Since when is developing and filling out a main character "filler"

But... this is almost exclusively how filler is used. Filler is really just code for "not integral to the plot," i.e. could be removed with no impact. Tbh Frank and Bill was 100% filler. It didn't have to be, but they chose to kill off both the characters and leave the location pretty much useless, so none of it will ever have any bearing on the story. The only purpose that episode had was to get Joel a vehicle, that's it. The rest of it was, by definition, filler. If for whatever reason they had to go back to Bill's town, then parts of their story can be used in the actual plot but as of right now it seems pretty obvious that won't happen.

The case is the same for this episode; no characters were introduced that might be useful later, the story is entirely self contained and only served to make the audience empathize with the character more. It could be removed without any impact on the plot -- aka filler. The only thing that happened in this episode really is that Ellie stitched up Joel. Sure, there are pipe bombs in the mall but we're not going anywhere near that QZ again so it's completely irrelevant. You could skip past this episode straight into the next one and I highly doubt you would even notice.