Mxy explains in a comment on that question quite clearly what the distinction is:
[...] it asks for a method [...], as opposed to asking for a specific product.
This makes it not a shopping question. It's not an "acceptable shopping question," it's "the 'good form' of what would otherwise be a shopping question." (emphasis mine)
It's as simple as that, and a basic premise of good Stack questions everywhere: we should describe the problem and ask how to solve it; we shouldn't decide ahead of time the kind of thing (like a system or app) we expect will solve the problem and ask what within that category we should use.
For more specific guidance on how to ask for help with a problem in a non-shopping way, check out last year's "I want to ask for recommendations, but that's off topic. Is there any way to ask my question?".