Na, a non sequitur is just any argument or conclusion that doesn’t logically follow from what preceded it. IMO it has no connotation of “suddenly” or even that it relates to irrelevance like segue does; rather a flawed argument. Whereas a segue is a transition from one topic to another separate topic without interruption, a non sequitur is clearly connected to what preceded (ie the same topic) but is simply illogical.

“I bought you Shiraz wine because you’re a receptionist. My cousin is also a receptionist and Shiraz is her favourite drink”.

vs

“I bought you Shiraz wine. Do you like being a receptionist?”

It does get used a lot in comedy where the logic linking the two statements is ridiculous, making it seem like two unrelated topics sometimes. It’s still just a flawed conclusion on the same topic though, it’s just taken to an extreme to make it surreal.