In the case that a new user has a salient comment to make that is not an answer, but has not the ability to comment, is an answer a viable alternative?
Answers are for answering the question.
New users should try to post actual questions and answers until they have the measly 50 rep needed to post comments. As mentioned in the linked answer, this is not a discussion site.
If the new user's comments can be reformed into an actual answer, then they can post that, and add their other thoughts as a comment to their own answer (you can always add comments to your own answer, your own question, and any answers to a question you asked).
"Welcome to RPG.SE. This looks like a comment and not a complete answer. Could you restructure it into being an answer? When you've answered a question or two, you will have enough reputation to leave comments. We're eager to have you share your knowledge and insights."
Honestly, one of the few things I find discouraging about SE for RPGs is the mechanical feel of rejection of our new users (negative vote totals, etc.) Anything we can do to soften the transition from message boards to this format would be immensely helpful to our growth.
Just flag it as a not an answer. Or use a custom flag that indicates it should be converted to a comment. In this case neither is really a good choice here so you're move of leaving a comment indicating the answer was inappropriate was the way to go.
The only real advice we can give to someone is that they need to do their time posting answer, asking questions and editing until they have enough rep to comment. Commenting is a privilege not a right and the rep to earn commenting privileges is low enough that it should only require a post or two around here.
As a new user, I find it completely baffling that I can't comment. Doubly so because (unlike with downvotes) it's not obvious why. I initially thought I just didn't understand the UI.
If the rep required is "measley", why is it there at all?