He’s not but you didn’t read the decision or his dissent, did you? I read all 42 pages and he dissented for two reasons.
The first, because the majority departed from historical precedent and he doesn’t think they gave a reason for doing so, and would have stuck with the past precedent. For this reason, he’s totally right about the majority not addressing past precedent, but I totally disagree with his belief they should have just stuck with it.
The second reason was basically a devils advocate argument, and he makes a great point. The majority opinion doesn’t do what you probably think it does. I’ll just quote Thomas here.
“In effect, it states just one rule: Schools can regulate speech less often when that speech occurs off campus. It then identifies this case as an “example” and “leav[es] for future cases” the job of developing this new common-law doctrine. But the Court’s foundation is untethered from anything stable, and courts (and schools) will almost certainly be at a loss as to what exactly the Court’s opinion today means.”