indeed, many Thais have a certain Chinese anchestry or at least claim so (in particular those in the business elite). Many large Thai companies have been founded by Chinese immigrants, but this was usually 2-3 generations ago.
As they married with locals, some families do not speak Chinese anymore at all and some speak, but mostly dialects (like Taechew) and not Mandarin.
However more and more private schools (and all Thais from middle and higher class send their kids to private schoos) teach Mandarin and for some, Mandarin is becoming more important than English