I think DaasYochid is referring to this:
http://www.hebrewbooks.org/pdfpager.aspx?req=14679&st=&pgnum=190
According to R’ Moshe, it would come out that some hold that Machnisei Rahamim is not a problem (and presumably that it is not included in the Rambam’s ikkar). However, those who were against Machnisei Rachamim, presumably are saying that it is against the Rambam’s ikkar. If so, then the only way that the forbidders could not consider the permitters as heretics would be if they say that the Rambam’s position is not necessarily binding. Which according to my arguments above, wouldn’t even make sense – if it’s heretical then it’s heretical. Maybe they think it’s a safek? I don’t know.