Nope, the battery may still be problematic, or there may have been bad batch of batteries at some point. It could be that the battery is more susceptible to damage by some design fluke. I personally don’t believe the comparison with feature phone batteries in terms of lifetime is as straightforward as it may seem, hence the comment about that above.
In terms of hundreds of cases: I’d actually expect there to be > 700 cases so far, based on 60.000 phones out there + replacements for normal battery wear over time and a 1% manufacturing defect rate (which has previously been cited as acceptable as for DOA’s for a smartphone as a whole here - the rates for failure in the first year are higher, so it’s a conservative estimate). If it is a manufacturing defect, that’s exactly what the warranty is for - you get replacements/repairs for manufacturing defects for the period the device is guaranteed. That’s still not counting the cases where the battery was replaced after it was mistreated (dropped / exposed to heat / etc).
If it’s any use, they changed manufacturers (and probably design, given a higher capacity) early this year / late last year (probably as all manufacturing switched around when they went with a different partner for the FP2):
Full disclosure: I must be very lucky - I’m still managing by charging the phone once every two days (I could probably stretch it to longer) on the original battery (and yes, I’m one of the first 5000): it’s held better than I expected it to, given the use it gets.