> The column heading just says "mix ideas", I don't think they're supposed to be taken as finished, stand-alone jokes.

Right above the table there’s a title that says “Joke Ideas”, and the text introducing the table is “And here are a few jokes that were created using this method”.

Everything indicates they are intended as jokes.

> Take the not entirely dissimilar Windows Vista joke from the IT Crowd

That joke doesn’t use the “mix ideas” concept at all. It’s straightforward in not deviating from the theme.

> It's not especially funny if taken in isolation, but in the context of the scene and the show as a whole

The idea of that joke could’ve worked in any context. It’s just:

A: “I’m having a computer problem.”

B: “What OS is that computer on?”

A: “Windows Vista.”

B: “That explains the problem.”