It's based on Matroska. But it's a different thing. Matroska itself still isn't used commercially.
It isn't just "based on" matroska, it literally is matroska. Call it a subset if you want to be pedantic.
An incredibly limited subset for sure. WebM with VP8/VP9/AV1 is a far cry from how Matroska is typically used, with supported codecs like h.264/h.265 and chapters, subtitles, multiple audio tracks, and linked external files.