BS. He was using copyrighted Unix code in his class prior to Minix ever existing.
He was using a textbook that had UNIX code and taught UNIX internals. He wrote Minix from scratch exactly because AT&T declared the book illegal, because it was no longer legal to show anyone AT&T code. He needed legal material with which to teach his classes. It was published openly in a textbook, read by millions, no complaints so far.
Again, produce proof of Tanenbaum stealing AT&T code or shut up.