Catholicism delineates itself, from basically, most of the rest of Christianity. It purports that they , non-catholics/, literally aren't saved, etc.
Hence, even aside from differing traditions, or whatever, they have actually defined themselves, as a different religion. In this sense, if you say they ''are'' Christianity'', then that would mean that they are the only actual Christian group.
If you don't believe that ,/I don't, then they are a 'different religion', by default. Self definition.
As to the issue of the word 'Christian', it could be argued that catholic church is actually promoting less of a Christian concept, and more of a 'church' concept. This argument could take different ideas, but it may not even be necessary