That could be true for most people convicted before Megan’s Law. Since there was no registry then (excepting California’s), people could not be sentenced to register.
HOWEVER, since the introduction of the registries, typically a person is told at the time of their conviction that they are required to register, so the justice system (namely SCOTUS) would treat the registration requirement as part of the sentence. Even the dissenting justices in Smith v. Doe said they would not deem a registration requirement to be unconstitutional if the requirement was ordered at sentencing. The same would be true for the restrictions that have been deemed “punishment” and therefore unenforceable ex post facto.