However, I'd feel like a punk if I'd take the linked article, write the answer myself and then accept my own answer to my question.
Do it. It'll be beneficial in every which way.
If you want them to write it themselves, give them a day or two to have a chance of doing so. But if they refuse or show no signs of doing so, go ahead and create an answer based off their comment if you want.
You'll be rescuing information. That's good.
Comments are going to be deleted, no matter how valuable they are, unless they're doing what comments are for: requesting improvements or clarification. It would be a shame to lose that information, so someone should recover it in an answer if the person who posted it doesn't want to.
So if you want an answer, go find it, then share it with us and get some unicorn points for it. Go ahead and claim the credit. You did the effort of crafting an answer; you earned it.
They either won't care (good) or they will (also good).
If they don't care, everything's dandy and we don't need to explore any further.
If they do, though, maybe that'll motivate them next time to actually make their answer an answer if they care to — if they wanted credit for an answer, they should've created one. If they don't want to or don't care next time, they'll probably just post a comment as usual and we rinse and repeat.
If they complain saying they were in the middle of their own answer, tell them to post it anyway. If they do (with or without complaining first), you can both reap whatever votes your answers have earned and enjoy that. (If you feel so inclined, you may also delete your post to "make way" for theirs, but I personally recommend against doing this - sometimes people will find your answer more helpful than the other.)
If they complain outright that you did this at all, that falls under the "they do care" path of this section.
A healthy attitude of snoozers losers works fantastically overall for dealing with answers-as-comments, so go ahead and employ that attitude and turn comments into answers if the author won't do it themselves.