Strictly speaking he's incorrect. Modifications are OK as long as it can be demonstrated that the modification was being done within 10 years of the end of production. In the case of escorts, Ford offered a roll cage option and many thousands were fitted by the end of 1985, so going by the DVLA's own guidelines, they are allowed.
Furthermore, modifications made purely for safety reasons are allowed. It would be hard to argue that a roll cage has a purpose beyond safety.
Despite that, some people have had problems. Some have rolled over, others have fought the DVLA legally, and won.