Gibson's logic is far from airtight. What Thuc did would be more like what Pope Liberius did when he persecuted Athanasius under duress.
Even if he was a heretic, and he very possibly was, considering almost everyone at that time believed in NFP and that you could be saved in false religions, his Holy Orders are still valid.
The Thuc line is actually more compromised by certain Thucites' association with Old Catholics who trace their Holy Orders back to a man excommunicated BY NAME by Pius X. This is the case for all those at CMRI who served under Francis Schuckardt, who had Old Catholic Holy Orders. He may have excommunicated himself by having himself consecrated that way, and all who went along with him may have done the same. But perhaps not, due to epikeia. When they abolished Schuckardt, the affected clergy of CMRI also had themselves conditionally re-consecrated or re-ordained.
I think the Thuc line is fine in terms of their validity and even liciety, due to the aforementioned epikeia, as is the Lefebvre line. The problem is that both the sedevacantist Thuc clergy, and the SSPX clergy, are drowning in heresies. This is what most people do not want to face.