On the website and even here, there is no advertisement of an endorsement or use of the author's name for this product. In fact, it is an attribution.
We already know they took the open-source code, re-implemented it and closed-sourced it into a product and that is permitted under the MIT license:
>> So you vibe ported manim to rust, kept it closed source, and you’re selling access to it with a monthly subscription? This isn’t satire?
This isn't the same as what you would get from something like Markdown [0], which explicitly says:
"Neither the name “Markdown” nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission."
What he must do is to attribute credit of the library to the original author (3Blue1Brown) for the derived use. But other than that, this is permitted.