People who unironically use the word "tankie" would say it's someone who uncritically supports communist governments, but if you ever look at someone using it, it's always because they are uncritically against all communist governments and so they interpret nuance or historical understanding as blind loyalty to communism.
Yeah "lesser evilism" always supposes the choice is being made in a vacuum where there's only 2 options and nothing can be done about it later, there will never be another choice.
Obviously if you were presented with two options and that was it. You would always pick the lesser evil.