By your definition, every community is a tankie because every communist rejects idealism. If these are the only two options, the only option left is to choose a team. But that can’t be right because you imply that some communists aren’t tankies.
Further, does it count as a definition if other people use the term in different ways?
If so, how do you know who is a communist and who is a tankie without asking them how they decided to show (critical) support for XYZ?
By your definition, you must first know whether someone has strong reasons to support XYZ before being able to decide that they really decided because XYZ was on the right team. That would be exhausting and fraught with the problem that nobody is going to say they didn’t do the reading; if they give an argument, how do you determine whether it’s valid or a cover for ‘choosing by reference to team’?
I’m unsure if it’s possible to define ‘tankie’ by reference to ‘communist’ without also defining the latter and showing how they’re different.