I think you hit the nail on the head with this comment. Stalin was a very influential man who shaped large part of the 20th century. Villanizing or idolizong his achievements without acknowledging the other side of the coin would be having an incorrect outlook on him.
I took a quick read of the link describing tankies. It more or less echoes what you said. That being said my observation of the use of the word tankie doesn’t fall in line with what the author was talking about. I’ve seen it used primarily for people who staunchly or blindly defend figures like Stalin and are incapable of acknowledging any criticisms of said figures. What yoyre describing is more of a lefty or a socialist in my opinion. The article was written in 2020 so maybe the use of the word has evolved over time. I haven’t been familiar with the word for that long to say otherwise.
That sounds to me like you were just dealing with bad faith actors, which isn’t uncommon here unfortunately.
I think we both agree on what it’s intended use is meant to be for. I guess you’ve just had the misfortune of dealing with people misusing the label to shut down any actual discussion.