Another phrase for Tankie ideology is Red Fascism.

I'd specifically like to highlight that someone who was imprisoned in a Nazi concentration camp - so has firsthand experience with a fascist regime - described the Soviets as the following, emphasis mine:

Kurt Schumacher, who was imprisoned in Nazi concentration camps, but survived WWII to become the first post-war SPD opposition leader in West Germany, described pro-Soviet communists as "red-painted fascists" or "red-lacquered Nazis"

So in what ways exactly is a tankie not a fascist?

a political philosophy, movement, or regime (such as that of the Fascisti) that exalts nation and often race above the individual and that stands for a centralized autocratic government headed by a dictatorial leader, severe economic and social regimentation, and forcible suppression of opposition

that exalts nation

"We did not hesitate to shoot thousands of people, and we shall not hesitate, and we shall save the country." - Vladimir Lenin

often race

https://www.britannica.com/event/Holodomor

that stands for a centralized autocratic government

The State

headed by a dictatorial leader

Lenin;Stalin

severe economic and social regimentation

Five year plan

forcible suppression of opposition

Cheka secret police.

If Tankies are not fascists - then neither were Nazi's.