While your point that every step of a proof requires justification is a good one, this is really, really easy to verify. That doesn't excuse one from actually giving that justification, in the course of a formal proof; but there's at least as much implicit knowledge bound up in the linked proof. (For example, one needs the existence of sine, its 2π-periodicity, the fact that it is positive from 0 to π/2 ….)
And actually any periodic function that is zero somewhere could be used in a tiny bit modified version of this proof.
I think that, for the modification to be tiny, you need something like: there is an interval on which the function is positive, and on the boundary of which it vanishes. (Some sign condition is necessary; taking the constant function at 0 doesn't work! Of course, this particular condition is automatically satisfied for any continuous, periodic function that vanishes somewhere, but is not identically 0.)
Personally, I really like the proof using sines. I won't claim it is easier or simpler but it is very nice. I've never thought to prove the infinitude of primes this way and it uses facts from trigonometry. I will now incorporate this into my trig classes.
I think it is quite straightforward and easy for one to understand. Students in trigonometry and calculus generally do not have experience in proving statements about arithmetic. Here is an excellent way to show a connection between concepts that seemingly have nothing to do with each other. It is this that mathematicians really like.