From what I gather from the article, yes: the index is stored on your browser. From the post:

This means that when you have search message content enabled, but your web browser is closed, an attacker with physical access to your device cannot read the contents of the index file. If you are logged in with your ProtonMail tab open, a decrypted version is stored in memory, but at this point, they would have direct access to your emails, anyway.