Generally speaking the breeder knows the growth characteristics and assigns it a class according to those. A rose derives half of its genes from each parent, but as the parent of any child can tell you, one parent or the other usually dominates, lol.

Genetically it should be more than "half modern". If M. Tiller is diploid and Lilian Austin a likely tetraploid, Licorice Tea has probably inherited only one third of its chromosomes from its Tea parent.