When a communist calls themselves a commie, it’s a form of taking the insult’s power away from those that use it as an insult, and gaining solidarity from it, much like how racial pejoratives often come to be claimed by those communities in defiance of the original dehumanizing intent of the aggressors.
A capitalist calling a communist a commie is insulting. Just like a communist or socialist calling a capitalist a bootlicker.
A communist calling another communist a commie is solidarity.