subpanel breaker tripped as well as main breaker - should I be concerned?

Grats on your successful generator test :)

Whenever you have a bolted fault, you flow hundreds of amps. The only thing that impedes flow is the resistance of the wires themselves, which you can look up on the internet. "Hundreds of amps" is well within the range of the "instant trip" functionality of every breaker in the panel except perhaps the main.

Due to manufacturing tolerances and environment changes, "instant trip" has an acceptable range. The acceptable range runs between 4x and 10x, where it becomes a guaranteed trip.

So for instance, a 500A bolted fault is a guaranteed trip for a 15A and 50A breaker, and a possible trip for a 100A main breaker. Probably will not trip a 200A main.

If your short had flowed only 90 amps, both breakers would operate in thermal-trip mode (most likely). The 15A breaker would see a 6x overload, and the 50A breaker would see a 1.8x overload. The 15A breaker would trip in a few seconds at most, and the 50A breaker would trip in a few minutes, based on the usual trip curves for breakers. In that case, the "selective coordination" you were hoping for would work as you'd expect.

However, all bets are off in a bolted-fault/instant-trip mode.

Apart from your 15A sub panel breaker and 50A main panel breaker, there is the main fuse in the house entry panel, which is sealed by the utility company, and a fuse somewhere in the street. Your complete question would be if you should be concerned about tripping any of these FOUR devices.

The answer is that these protective devices should be coordinated, so that only one of those, closest to the fault, should trip. Remember the commercial where a hobbyist plugs in his new Christmas tree lights and the entire city goes dark? That is of course what nobody wants.

How high is the short circuit current? Ohms law says voltage divided by resistance. There is resistance in all wiring between the transformer in the street up to your short circuit location. Also, add in the impedance of the secondary winding of the transformer, and the resistance of the protective earth conductor. The value of the short circuit current is by design pretty high in order to trip the fuse or breaker fast in case of a short, mainly because the earth connections should be low resistance. But not too high to avoid the fuse or breaker to explode.

Does coordination work? Most of the time. In your case, the 50A breaker trips just as fast on that short circuit as the 15A breaker. Circuit breakers can trip really fast, and the ratings are 'only' a factor of 3 apart. Fuses may be a bit slower, so you did not have to call your utility to replace that one. But if you toy around too much, you may be out of luck here.

Should you do a test on your two breakers? If you already called yourself a dummy, messed up with electrical tape, and also need to ask the question, I'd say you would better leave this to a specialist :-)

I know something about electric but am not an installation specialist, but I doubt that new house installations are routinely tested with short circuits. Coordination is designed with short circuit calculations and by carefully evaluating the specs on the circuit breakers. Measurements are done in high tech labs by circuit breaker manufacturers.

Due to there being two sources of power it's difficult to say exactly what happened. Here are a couple of possible scenarios:

1. 50A breaker tripped, cutting power to panel. Generator started, transfer switch activated, short-circuit continued flowing current. Now the 15A breaker trips.
2. 50A breaker and 15A breaker were both heated concurrently by the short circuit. Due to manufacturing tolerance that particular 50A breaker might respond a little quicker and the 15A breaker a little slower than nominal. If so then for this particular fault it could be that both activated their magnetic trip trigger at approximately the same time and therefore they both opened.

In general, breakers are guaranteed (required) to trip when their rated current is exceeded for specific time (Different types of breaker - [A,B,C,D in EU] - have different curves). For this reason breakers are arranged from highest to lowest rating, so the smallest one reacts first.
But
In case of short, breakers may trip in any order, including higher rated breakers up the chain. Short is basically operating outside specs, so no order is guaranteed.
[Personal experience: After "finding" a cable with angle grinder, the light breaker popped, then after couple seconds of buzzing noise, a main fuse popped as well. Had to break utility company seal to replace fuse and call them to retag the fuse.]

Answering your concerns:
Yes, this is normal when dead short happens.
You can reset both breakers after ensuring the short is fixed.

This in general indicates poor breaker coordination. It's a fairly involved topic, with books written about it. In general, with a properly designed electrical system, it shouldn't be happening. But you may end up having to buy parts that don't come from Home Depot.