The difference is that belief in Yoshke is 100% incompatible with Judaism. Even a Jewish atheist can be said to be practicing Judaism in some way, but an idol worshipper, absolutely not.
Reform and Conservative Jews who don’t keep the entire Torah aren’t just keeping what they do keep because “they like doing it for whatever reason”. It’s not just for fun. Many of them believe that they are mandated by halacha, some do it because it helps them feel a connection to HKBH, and others do it to connect with the heritage. All of those are valid reasons and are very much connected to Judaism. Even if you believe that there is only one right way to practice, that doesn’t mean you have to deny your brethren a place at the table. You might strongly disagree with their lifestyle, but that doesn’t warrant denying that they’re a part of the tribe. Reform rabbis wake up every morning and put on tefillin, daven, learn, officiate at funerals, teach Torah, and promote chessed. If that isn’t “practicing Judaism,” I don’t know what is.