Just be honest and say "please don't think I'm being rude, but I really need to get on with this work." You can be assertive without lying or being nasty.
I wouldn't use the tool in question, but I relate to the motivation.
So as others have suggested the response can be prioritised according to importance and urgency.
While flow is critical for productive work, it should be directed to the highest priority tasks.
please don't think I'm
being rude
By saying that you remind the interlocutor that being rude is in fact an option (indeed a widely used one) and you may simply be rude ... ah the subtleties of communication.Trust me on this, I'm not lying to you ...
People tend not to think themselves or what they're doing as important for whatever reason.
It also sets the scene for future interactions. Oh and sometimes it really is important..