Functional programmers work with infinitely recursive structures and data all the time and it doesn't really bother them, so this isn't automatically an absurd thing to say.

Just because structures, rules and trees are infinitely recursive doesn't mean no insight can be created from them in finite time.

Other people are explaining why you're wrong, but I'd like to point out that even if you were right, your comment would still be nearly worthless.

Merely pointing out that someone else isn't correct might even have negative net value. For one thing, it can be construed as simple disagreement, opening up a debate that shouldn't happen when there really is a definitive answer. For another, it's insulting (especially as you did it) and that's not a necessary part of correcting someone's understanding.

Invest a few more minutes next time to actually make a substantive comment. You'll be helping the person you correct, you'll be helping whoever else reads the thread, and you'll ultimately be helping yourself – by opening up your views to an actual constructive process.

Not necessarily. One can use tabling (memoization) to prevent that.

Also sometimes reordering clauses in the body of a rule is enough to avoid infinite runtime.

Datalog (or SQL + recursive CTE) don't have this problem.

Mozart/Oz or Alice allow to specify the search strategy.