To say such attendance is a mortal sin depends on too many circumstances to make a general rule. Three things are required for mortal sin, grave matter, awareness of the gravity of the matter and full consent. If these are all there it is a mortal sin. But let us say they are not all there and it is not a mortal sin. Does that mean God wants me to go to the NO Mass? I think not.
Are you suggesting that the confused trad who asked the question to TIA, does not recognize attendance as a grave matter? Or is unaware of the gravity, or would go with only partial consent (thereby nullifying the danger)? If that was the case, then why would he ask the question at all?
According to those three things....if he was planning on ever going there at all, the absolute worst thing he could ever do was ask that question. Thankfully TIA gave no answer so he remains confused, and because of his confusion he can call upon those three things to come to his defense should he need to - he is therefore free to go without any concern whatsoever about whether it is a mortal sin or not. And per his confusion combined with those three things, it's not a mortal sin.
Those three conditions are in need of correction because per those conditions, in his confusion he could decide to go once, then twice, then frequently - and lose his faith same as the multitudes who go / went there have. So much for those three things required for mortal sin.