My guess is that bridge is run as a service. i.e. you make reuests via web site which are encrypted with local javascript (so that proton mail can't interpose)

My guess is that bridge is run as a service. i.e. you make reuests via web site which are encrypted with local javascript (so that proton mail can't interpose), protonmail forwards to the service user runs which provides an encrypted response which local javascript then decrypts and displays. In that case, you have to run a service that is always running that is effectively keeping the e-mails available in the clear. Is it better security than keeping them in the clear on a central server? sure. But is it really any different than running my own smtp/imap/webmail server on aws without encrypting any emails which I access? In practice it would seem to be a similar threat model and I'm not convinced many would view that to be particularly secure, just a tad more private.