It doesn't look like any rot to me. In fact, I see a lot of healthy new growth. The problem with tail biters is that there are times when they've destroyed it enough that it never grows back proper. My last betta never regrew his tail like it had been prior to biting in the year after he stopped biting before he died. It was always just a bit ragged looking even though there were no new bites. I wouldn't medicate with anything as there are no raw edges and the clear new growth looks healthy.
Rollo*Male Plakat - Napoleon*Male DeT - SIP 2014-17 - Charlemagne*Male Butterfly -SIP 2015-18