"is not affected" being the operative wording. users want to know if their data has ever been at risk. still, surely everyone can just assume it was affected, act accordingly, and move on?
After a small amount of research, it looks like they run Java webservers, along with (or on?) F5 Big-IP platforms, with the later likely providing hardware SSL decryption that isn't vulnerable to Heartbleed (mostly, apparently there were some vulnerabilities in certain configurations where it would fall back to Open-SSL.) The way Java webserver allocates memory is also different that the typical Apache/Linux server, so it is unlikely that even if the server was vulnerable that a hacker would actually be able to pull any data of any value from the chunks they could get.
I don't profess to be an expert on server security or the F5 Big-IP platform, but my point is, it would appear that there is no reason to not believe Mint when they say they investigated and have no reason for concern.