Does it make sense to apply for assistant professorship positions before Ph.D. graduation?

Most people apply for positions while still students. That is normal. The "date of hire" could mean various things, depending on the institution. Don't worry too much about it, and apply. If there is any sort of issue they might be able to finesse it a bit, but likely they intend that early 2021 graduates are part of their intended outreach.

The date of hire is not usually interpreted as the date you get an acceptance letter, but the date you actually become an employee (with a paycheck).

Talk to your advisor.

It is absolutely possible for a person to get hired for a tenure track position straight out of their PhD (and when that happens, the person must have applied before they graduated). I know several people who had that happen to them. So in that sense, you are overthinking things. However, I’ll repeat: talk to your advisor. Just because something is possible doesn’t mean it’s likely, and the likelihood and difficulty of securing a position in such circumstances will vary wildly between research areas, countries, and different tiers of universities in any given country.

So again, sorry to sound like a broken record, but you’ll have to ask your advisor to find out whether applying to such jobs makes sense in your situation or not.

The job listing says

These positions require a Ph.D....at date of hire

So the requirement raises one question: Will the applicant have the degree, at the date of hire?

But if I act before graduation, will my potential application be considered when I have not yet sealed my Ph.D.?

Yes, your application will be considered if it notes that you will have your degree on the date of hire because you will then satisfy the requirement found in the job listing.

This obviously makes you responsible for sealing your Ph.D. If a job applicant asserts that the applicant will seal a Ph.D. then whoever assesses that applicant's application will assume the applicant is correct.