I wonder if he had any direct observational evidence to support his claim. Personally, I think he was mistaken, as I have known people who thought in different languages than myself, yet never noticed anything out of the ordinary (quite the opposite, in fact). Expressing thoughts in another language can come across seemingly queer (sideways is how I sometimes think it). There are words, even meanings of words, which may be absent from one language to another but generally there seems to be a way to arrive at similar conclusions, so I think they way humans think is similar. Culture can skew outcomes, but I think the process
leading to those outcomes is basically the same.
I would be cautious not to mix what I see as apples and oranges (cultural differences vs thinking processes). That’s my 2-cents, for what it’s worth.