つつある is used to show that a physical process is still ongoing. 帰ってる means someone has already returned home, not that they are in the process of doing it. 帰りつつある means someone is in the process of returning home.
Verbs that happen at a specific point in time need to use つつある to show that someone is in the process of that verb. E.g., 死んでる means someone is dead but 死につつある means someone is in the process of dying, normally implying a slower but inevitable process. You’d need to say 死にかけ to imply someone is about to die but isn’t fully dead yet.
Equally, つつある isn’t used for everyday activities.
Edit: Okay I have now gone and read the Bunpro entry and I think the confusion comes from the fact that Bunpro is trying to say that like everyday motions do not use つつある however they fail to mention that つつある is used to express ongoing processes with verbs that are binary, that happen at a specific point in time, such as 帰る or 死ぬ (you are either home or not home, dead or not dead). I think then what has happened is that the hint during reviews is saying that in the case of physical actions the emphasis is on being underway. This is in contrast to verbs that don’t describe physical actions, not saying that it is exclusively used for physical actions.
Basically the original explanation is missing some key information and then the hint doesn’t make sense if you don’t know that extra information.
To make the meaning clearer here it is best to not think of つつある as just a fancy version of ている. Rather, consider literally that it is つつ+ある. I.e., if someone says 帰りつつある they are saying that the 帰りつつ (the ongoing process of going home) exists, hence why this is acceptable (and necessary) for verbs like 帰る、死ぬ、開く、etc which can’t use ている to show an ongoing process. I hope that makes it somewhat clearer although perhaps I’ve just muddied things.
Either way @Asher, perhaps it is worth adding a bit more to this grammar point explanation and/or editing the hint.
