Not an expert on this, but maybe this is because you have been hosting your server for 20 years? Maybe newer servers have a higher threshold to cross? Seems a logical hypothesis to me, which would mean you're both right.
I selfhosted my email server on the same IP address for 10 years and never got my reputation tracked. I remember once reading that they needed something in the range of a few thousand emails going to their servers. I swear it used to be listed on the google dashboard but like everything else email related they hide this information to "protect from spammers". Sounds like an excuse for anti-compeditive behaviour to me