Not sure if the comment was edited or not, but "/s" is often used as a shorthand for indicating sarcasm due to it not being easy to infer tone in text, so "/HEAVIEST-S-POSSIBLE" is intended mean that you should read their comment in as sarcastic a tone as possible. So yes, they were being sarcastic.
Thanks for clarification. That was new to me.
The “/HEAVIEST-S-POSSIBLE” is an extension of “/s” which is used as an explicit indicator of sarcasm.