Just doesn't work, ads keep playing.

Works as intended! It doesn't block them, just mutes the tab while they're playing; you hear silence for the duration of the ad. Spotify detects ads are still being played, so your account is not in danger. Hopefully Spotify doesn't have the ability to detect sound at tab level.

Edit: I wasn't sure what "requires calibration" meant but looks like I have to click on "restore default" every time the web player is opened. Very minor inconvenience which I don't mind.

It works if at Options > Ad Title you fill it with how "advertisement" is said in the Spotify's language you chose, browser version (URL: https://open.spotify.com)
Keep on, how peaceful now! :)

I have been using this extension for a couple a hours and it seems to be working well, without any drawbacks. Thanks !