It’s one of those “all butterflies are moths, but not all moths are butterflies” issues. Butterflies are pretty specific, but moths are essentially anything within the larger clade that’s not a butterfly.
Mangroves are different because they’re ecologically categorized and share distinctive characteristics as a result of convergent evolution. This results is something that you can look at and pretty much anyone can say, “Oh, that’s a mangrove,” based on the very obvious shared characteristics, but those mangroves all come from very different families.
They are so unique that it’s difficult for even people familiar with them to place them in the correct families, but they share such unique characteristics that they cannot me mistaken for anything other than “mangroves”.
This leads to an extremely frustrating situation where it’s very easy to narrow them down to a small subset of potential families in real-world practice, but that there is absolutely no way to do so within the confines of inat.
As @tiwane says (and as I said in the post), there isn’t a taxonomic category that captures this, but this is a somewhat unique situation that’s not shared by many other groups and it doesn’t seem to me that it would “open the floodgates”.
Off hand, the only other group that comes close with regards to this problem would be “succulents” or “trees”. In both of those situations a similar issue arises; plants from many families have evolved succulent characteristics (or a ‘tree’ form) and are unmistakable as such, but there is no formalized way of referring to them collectively, despite them being instantly recognizable as a group.