As others hsve pointed out, it's a very coarse (and rather arbitrary) categorization.

E.g. both Turkish and Russian are in Category 3, but Turkish is trivial compared to Russian.

Turkish grammar is extremely regular, and follows easily defined rules that fit about two pages of easily digestible tables.

In comparison, Russian is a separate class tought in Russian schools for four years to native Russian speakers. And you still get people who can't properly inflect numerals, for example.

Isn't English also a separate class taught in English schools to native English speakers?

Not for four years, for all eleven years...