I guess part of the confusion is that it all seems to be a recent development. I didn't even know about the interspecies tag until I saw it cropping up in a few recent Pokémon pictures that were previously tagged bestiality and thus previously blocked by my blacklist. Hypno and the Machoke line seem to be the biggest ones hit; they're no longer to be tagged bestiality and instead interspecies?
But, even then, your centaur examples pose even more questions. post #3534276 is definitely not a furry as you state. In fact, of the few instances of centaur+furry being a tag combo, only three images (post #3279728, post #3279729, and post #3279766) appear to be tagging the centaur itself as a furry. And those three examples appear to be incorrectly tagged furry.
Again, part of the issue is that none of this appears to be currently implemented in practice. A majority of instances of tags that follow the guidelines you posted are tag updates from the past few weeks. Someone further up said "instead of a mountain of them." to fix, but there is a mountain of them to fix and I just wanna make sure my ducks are in a row before I start countin' 'em.
The question it raises is what of other Monster Girls? It would seem that post #2228208 is now to be tagged as bestiality. Do I also have to tag post #3187571 as interspecies? post #2431091 would be furry and interspecies, correct?
Granted, this is all in the wrong thread, so we should probably move to the how to tag thread, but still. Had to ask.