I'm sorry what is the joke? I feel old now for not getting it.

A "reply guy" is a pejorative term for someone (usually male) who consistently replies to posts where their opinion is not valued or wanted, or often inappropriate:

https://amp.knowyourmeme.com/memes/reply-guy

The joke is that the people selling this software picked that as the name for what whey are selling, either missing or leaning into the negative connotations that are attached to that term.