It's more a model of what a really talented person who applies themselves building things they enjoy building can do.
I prefer thinking of it this way: if Bellard can make a small JS engine from scratch by himself, what's really stoping you from knocking down this library you are thinking about.
Why do you say that specifically is his specialty? He also started QEMU and ffmpeg which are foundational pieces of software for several industries, and his day job is as founder of a company that makes software defined radio test equipment for cellular networks. There isn't one thing I could point at as a specialty.
Ffmpeg uses a ton of arcane C and assembly knowledge to make multimedia system manageable and efficient. QEMU uses dynamic binary translation for hardware emulation and virtualization. Amarisoft speciality is basically using software to do things which are usually done by hardware.
The intersection of programming language and system programming seems to me like a pretty fair description of what Fabrice Bellard is extremely good at.
That said, judging by the license file this was based on QuickJS anyway, making it a moot comparison.