"I think this is because a lot of filler in anime is generated by the producers who need to buy time until the manga catches up with the anime, so people learn to associate the concept of filler with this kind of shit. " This is quite literally the definition of filler. Anyone who uses "filler" to simply mean "content that doesn't advance the plot" is using the term incorrectly. if a beach episode appeared in the original manga, it's not filler!
"I think this is because a lot of filler in anime is generated by the producers who need to buy time until the manga catches up with the anime, so people learn to associate the concept of filler with this kind of shit. " This is quite literally the definition of filler. Anyone who uses "filler" to simply mean "content that doesn't advance the plot" is using the term incorrectly. if a beach episode appeared in the original manga, it's not filler!
I see the term being used for both, given the overlap.