When a tankie says they’re not imperialist, they’re generally referring to the strict definition set forth by Lenin, and not the looser definition in common use.
You’ll get some online tankies who dick ride Russia for sure, but they’re just larping. Real Marxists, who do real organizing, are generally more discerning.
Don’t get me wrong though, I’m not supporting them. They intentionally misunderstand when people say imperialist just so they can argue Russia isn’t. No one is talking about the strict definition when they say it to a tankie.
If you want to understand fully how they decide imperialist vs not imperialist you’d have to read, imperialism: the highest stage of capitalism.
Really it comes down to finance and the export of capital. Lenin differentiated between finance and exporting capital, and wars of territorial aggression. What Russia is doing would fall under war of aggression and not strictly imperialism according to his definition.
https://www.struggle-la-lucha.org/2023/08/16/five-key-features-of-imperialism/