thats why this TDs are only good counters/ or softcounter.../ counters.
Not HARDCOUNTERS. (once more: hardcounters means it will beat the unit in every situation and no lose possible: like a stone will ALWAYS lose to a paper.EVERYTIME. no lose possbile to the paper.)
Do you mean there are no hardcounters in this game? Because the only thing that for infantry is literally impossible to destroy are tanks. Which would mean that a Firefly hardcounters basically all infantry.
If you go for a 'on paper 100% win chance' I can understand the definition, but find it unsuitable for CoH due to the rng nature of the game. A T34 could frontally kill an Elefant in a duel, so there is no 100% win chance. On paper a P4 will win against a Sherman, but they're obviously not hard coubters.
So you could define a threshhold with maybe 90% win chance, but that would bring n subjectivity that you want to get rid of with your definition.
That's why I would go for intended roles which can be then subcharacterized by efficiency while always keeping a normal battlefield situation in mind. A Panther is designed to counter tanks and will win in most scenarios, but in certain setups it won't be the most cost efficient one. A Brummbär is intended to hard counter infantry, but maybe an Ostwind would have done the same job. That makes Brummbär still a hard counter, just a maybe less efficient one