Tracy Hickman, in the forward to CoS, says that "the vampire genre has taken a turn from its roots in recent years. The vampire we so often see today exemplifies the polar opposite of the original archetype: the lie that it's okay to enter into a romance with an abusive monster because if you love it enough, it will change...[Our hope in CoS is to] bring the message of the vampire folktale back to its original cautionary roots."

The "Lunch Break Heroes" video series goes into more detail on one way to run Strahd - as an emotionally abusive lover who systematically destroys Ireena's support network until she has no choice but to depend on him.

These are dark, adult, psychological themes here - and CoS is built to showcase them. It is still D&D, and the characters can still "win" and "beat the module" - but they should have a lot of losses along the way, and should see plenty of unpreventable death and suffering before they get there. Not every encounter is "winnable" in the sense of them feeling like heroes. In fact, most encounters should end up as compromises - victory means staying alive and slowly gathering resources and power for the ultimate confrontation with Strahd, but along the way they should experience plenty of tragedy.

So, I would suggest that the level indicators for the Chapters are not the levels at which the party can "win". Rather, they are indicators of the level at which the party can reasonably appreciate that they can't win. Or the level at which they can decide what they might be able to salvage from a bad situation. They are the minimum appropriate level for the party to experience the horror, dread and their overall helplessness to do anything about the pervading evil of the land. Or, to rally and decide that whatever they can do, whatever small victory they can achieve, is enough for now, will have to be enough. At any lower level then this, an encounter in the area will be over too quickly - they will simply lose without time to appreciate their helplessness, and the feeling will be one of frustration and disappointment rather than dread.

For example, at level 4 in Vallaki,

Likewise, they can probably

The module is full of situations like this - the PC's should have the possibility of marginal victories, but never complete ones.

Within this context, what does victory at Old Bonegrinder mean? A clever and careful 4th level party might be able to

I am largely in agreement with Dale M's answer said, but I do disagree about one thing. He suggested that the level might be more appropriate for combat if the party is able to separate the foes and take them out one at a time. I don't think fighting even just one of them, at least at the first level indicated, is feasible.

You say "TPK" like its a bad thing ...

There are groups that adopt a style of play where the onus is on the players to assess risk and to pay the price when they get this wrong. These groups scoff at your namby-pamby ideas of "level-appropriate challenge".

Curse of Strahd is explicit that this is an appropriate and encouraged style of play in keeping with its theme of gothic horror. Characters in Strahd are tragic heros - both the PC protagonists and the NPC antagonists - there is no escape from their destiny as the Epilogue makes very clear.

If this is an alien mindset then this module may not be for you.

The module gives guidance

From the Introduction (my emphasis):

The last highlighted sentence here can be a totally foreign concept to many players. As a DM, you need to be very clear that a) this is an option and b) you should use this option right now!

This is not a combat encounter

These monsters are more than willing to make a deal - they can probably make the PCs an offer they can't refuse given what they have to bargain with (and always assuming your PCs actually care about that). Just remember:

Faust's story is entirely in keeping with Strahd's theme of gothic horror.

These creatures aren't necessarily encountered together

There is an opportunity to divide and conquer which makes the whole thing much easier. There is a specific encounter with one of these creature's elsewhere and earlier. If your players deal with this one then, as mine did, the CR and the expected level are much more closely aligned.