Hacker News Reader

Does any Matrix client support voice/video calls other than Element (including Hydrogen as it shares Element Web's codebase)?

Fluffychat supports it too. Check out https://matrix.org/ecosystem/clients/, you can click the clients for a feature list.

Hydrogen does not share a single line of Element Web’s codebase.

I must have got it confused with another client then. Wasn't there another lightweight one made by Element-the-company that reuses the react SDK?

nope.