No way to know if the right thing was done or not.
IF he made a complete and genuine confession of his sins, and received the Eucharist, as SSPX states, then it would seem that he had a right to Last Rites and a funeral.
If it was still thought to be scandalous, then the funeral could have been held quietly in private.
Those are big ifs. The article also says that he never publicly repented his actions. That may or may not be true. Maybe he did not admit his wrongs at the trial, but repented later. And no way to know whether he made a good and valid confession, without a priest to testify as much.
He openly defied Church teaching throughout his life.
I would say that unless credible witnesses attest that he privately repented, he should not even be interred in consecrated ground.
That was what the Pope ordered: a private funeral, not held in a church. The SSPX decided to give him a church funeral.
I agree. It's awful, though, that this went on in the first place, and it's very unfortunate that it's not clear that the man had repented.
Ultimately, whatever rites were performed or not, God knows everything and decides his eternal fate.