There is enough evidence to say it's D&D 5e. We still shouldn't edit and reopen it without OP editing it.
I will refer to D&D 5e as only 5e for the remaining answer.
I will borrow from KRyan's and mxyzplk's answers to other meta talks about this. Even if the chance that this is not 5e is 0.01%, it does not matter. There are meta-reasons for following our current policy in a strict sense.
Note that our current policy only allows for editing/reopening if an explicit statement about the system is made. If you disagree, you can change it by up and downvoting answers in this linked meta question.
It reminds and teaches people that there are other RPG systems other than the one they are playing
Let us be honest. Our website is almost dnd5e.SE. Our policy reminds people that there are other systems around here. There are other systems/editions that have "player handbooks", there are other systems that use d20s, there are other systems with concepts such as HP, AC and with classes such as Fighter and Sorcerer. While probably there isn't any other system with all of the things listed in this question together (Wildemount setting, legendary resistances...), the mentality that "other systems don't exist" is bad for an RPG community, and we shouldn't take for granted that the system we are playing is the only one. One very strong argument made by KRyan:
People incorrectly assume D&D 5e all the time — even when questions are tagged with something else.
We don't want this to be any more reinforced.
Side note: a setting can and is easily translated to other systems, if it wasn't for the "legendary resistances", which AFAIK is very specific of 5e, I wouldn't even be sure about the edition. People have translated 3.5 books into 5e quite often, for example.
It teaches people about our policy
Straight-forward: people seeing the question being closed will learn about the policy. If enough people learn about it and disagree with it, we may revisit the policy and change it. So far, every time we have revisited it, the majority of people have agreed that keeping the policy in its current state is the best idea. We should not, however, try to enforce a change at individual level (e.g., by tagging the question and reopening it without consulting the community, as in this meta question).
It optimizes for pearls
Okay, this one may be a little too harsh, but as mentioned in one of KRyan's answers, I do not see value in an unclear question made by someone that doesn't care about coming back and making their question better. If the OP doesn't come back and doesn't edit the question with a very simple change, why are we so worried about giving a proper answer to that question? OP may not even come back to read it.
If the Q&A would help someone else, then this someone else may ask the question and properly define the system and be active to clarify any further doubts we have before answering. This is more likely to provide a good question to the site, with good answers, than a question where the OP didn't even come back to clarify the system they are playing.
In short, we should be enforcing questions and answers to be better, not simply accepting questions because we can deduce what they are asking.
We don't lose anything by waiting
And, finally, I don't see any harm in waiting for the OP to come back. Do we really need this question answered by tomorrow? Do we really need to internet points (a.k.a. rep) we are going to get from an answer to this question?
The only harm I have seen done is people not following the damn policy and starting edit and vote wars. That is more of a reason to simply follow the policy and wait.