MKV is a container format for bundling together streams of audio, video, and text. It does not provide the actual video compression, which is still typically h.264 or h.265.

The other answer you got hits it on the head already.

Here's an additional piece of info you might like: The webm container format is actually a specific subset of Matroska. The big players in the web recognised that it was a very useful open container format and adopted it for the web. They took a subset to make implementation in browsers easier and more uniform.