This is timeline is absolutely on target and our personal experience.
I thought the same thing. My own son who has sz now was very late in speaking and had to even have speech classes to encourage him to speak. He didn’t even talk in full sentences until he was almost 4 and language in the form of expressing complex thoughts and sentences has always been hard for him. I also suspect my son might have been on the autism spectrum but no family doctor ever brought that up. My son is 35 now.