Presumably this affects all electron apps which embed chrome too? Don’t they pin the chrome version?

Yes, but it's only a vulnerability if the app allows rendering untrusted HTML or visiting untrusted websites, which most Electron apps don't.

pretty sure I've had slack show me whole web pages without kicking me out to the mobile browser.

Except: Spotify (through ads), Microsoft Teams (through teams apps), Notion (through user embedded iframes), Obsidian (through user embedded iframes), VSCode (through extensions), etc...