Why are we using a past modal verb with a present verb in this sentence?

This is very likely a misprint.

The sentence as written is not grammatical. The most likely fix is that there is a missing word before "speeds". Good candidates for the missing word are "achieve" or "reach" or various synonyms.

This would make "speeds" not a present tense verb but a plural noun.

"Speeds of up to..." is a very common noun phrase.

There are other less likely solutions, which require more changes, such as "was able to speed [along at] up to three times [the speed of] passenger planes today.

Apart from a missing word, the past tense is used in

Concorde was able to [achieve] speeds of...

because it no longer flies. The 'feature' (ultrasonic speed?) is still used, so

it is typically restricted to military aircraft

using the present tense.