The original NORBA rules, which I wrote in 1983, did not specifically exclude cyclocross bikes, but the rules about no replacements and the selection of rough, high-speed courses effectively kept them from showing up at MTB races.
The UCI hadn't had to deal with the problem of MTBs entering 'cross races until recently, and they effectively rule them out by limiting the parameters of the bike's design. They didn't like us then, and they like us even less now since our sport took over the world and theirs didn't.
But MTBs were definitely competitive in any of the 'cross races I have seen them in, and a rigid frame like my P-21 might even be modified into meeting the current parameters.
Not that I would bother.