Should I write a comment paper regarding the following issue?
If your claim is valid, strong and holds for all possible kind of problems that could be supported by the lemmas given by the authors, then you should do the following:

As the paper has been accepted (you should wait. Because, the paper you read is the paper you read might be from author's personal sites or arxiv (as I assume)). So, once the journal/conference publishes the accepted paper online, you could write a comment paper/a short paper based on your claims.

Without online publication by the journal, it is really difficult for you to talk to anyone. I guess.