Maybe the implementation is smaller than a typical FORTH?

For instance the FORTH implementation I'm somewhat familiar with (on the KC85 8-bit home computer) was an 8 KByte ROM module, which was packed to the brim - and it wasn't a luxurious FORTH by any means. This Mouse compiler seems to fit into 2 KBytes.

IIRC they were roughly contemporaneous (forth was around longer, but mostly as something Chuck played with and most people didn't learn about until much later). In those days you couldn't just google for alternatives or hop on something like HN to see what other people were working on. So you wound up with all sorts of local variants on the same idea, and it was only years later (if ever) that people learned that they were working on similar ideas, unless 1) somebody published, 2) hallway chatter at a conference, 3) somebody knew somebody who knew somebody and you found out serendipitously. Very different vibe.