I can see how these small references can be described as the thin end of a wedge that might split apart the immersion for many people.
When I read clocks and pop guns in Tolkien (and velocipedes in the Shire supplementary) it can jar that immersion for me as well. But in fact, it doesn't actually derail my immersion, because the reference is not extended beyond that individual lapse into whimsy. I therefore find I can ignore these little oddities, or if that is not enough, some of these references I can think away:
1. Why do we always presume Bilbo's clock is a Victorian wind up spring clock? A water clock would not break immersion, and is perfectly low tech.
2. I would compensate for "like a pop-gun" by imagining Tolkien meant to write "with a pop".
3. Matches might be the very gift that Gandalf (the master fireworks craftsman of middle earth) donated to the hobbits in that bad winter: and so let them make fires readily to survive that time.
4. Velocipedes? I did that upthread.
The secret here is not to let the niggle destroy the story. If it is too difficult to incorporate something, I just drop it for interfering with the tale. And never let it spread out of the Shire.