However, calling it racist bein unable to differentiate between Asian languages is a bit farfetched. I mean I am a European and I can't distinguish most European languages, simply because I don't know them.
I don't think he is saying that an inability to differentiate between foreign languages is racist, but rather that given a random shithead who holds truly racist beliefs, it is unlikely they will be able to recognize differences between Asian languages.
To put it more generically, most people in Set A exhibit Trait B, but having Trait B alone does not mean you are a member of Set A.
He probably meant that her mother thought if they spoke thai instead of mandarin, racists wouldn't think about their "every asians speak chinese" stereotypes.