The whole point is that there isn't really any clear cut difference between "tankies" and "actual leftists" because it's a pejorative term that each person using it defines differently based on their own personal politics. Is critical support for Vietnam tankie? Cuba? Makhnovshchina? Ukraine? The Nordic Model? The Black Panthers? You'll never get consistent answers from people yelling about tankies.

one takes them seriously because the things they profess tend to be silly and they usually have that obnoxious College freshman master-debater energy.

Ironically, this better describes the people who rail against "tankies" better than it describes anyone else.