That Father Luther was flawed is evident. Yet God used him.
Every single Pope was also flawed - many outright evil.
Peter was flawed.
All the Apostles were flawed.
God has always used flawed humans.
That he used Father Luther to recover what Rome’s flawed men lost is not surprising.
Tradition, bringing everyone down to Luther's vulgar level to try to salvage him as the foundation. If you wish to source him in the scriptures, try Baalam/Balaam.