While you're correct that a 'worst-case-scenario' would be to judge total possible propagation time on the TTL of the record prior to it being modified, this doesn't hold true in a few cases:

  1. Every DNS resolver caching it will have accessed and cached it at different times, not just when you modified it.
  2. Your original TTL could've been 'too short' for some resolvers, and they substituted a larger value, which you don't know.
  3. Your TTL could've be too long and it expired, or cache was reset/filled, or cache too small, and it was dropped from cache.
  4. The operator of the resolver could've chosen arbitrary TTL to assign to all records.

So instead of guessing across these scenarios, it gives a 'sample' of how resolvers are handling your particular situation at a particular time, and gives some perspective into how other, non-reachable resolvers may also be handling things.

To one of your other points, keep in mind that many resolvers are stub resolvers, using an ISP's name resolver, and so these DNS servers queried here may be in use by much larger audiences than you imagine.

Its certainly not a guarantee of much information, but Imagine scenarios:

  A. You see that 90% of the returned resolvers are giving you the old record.
  B. You see that 90% of the returned resolvers are giving you the new record.

I'd consider that information of value in determining overall 'propagation' of the new record to resolvers.