WebP is recent enough that it shouldn't have even been a real concern. 16383×16383 even in 2010 was an absurdly small size. I've had PNGs and JPEGs with larger dimensions for longer than WebP has even existed.
I agree those limits are too small.
WebM is, strictly speaking, a container format that is a subset of Matroska. It doesn't have any real relation to WebP.