they could do like what mutiny wallet does and run most of the node in the browser, and for the rest use a web based proxy

How do you make something non-custodial without linking your own node? 

TheBTCManual

 commented suggesting that a non-custodial  option (without linking to another wallet) is coming to Alby.

lightning

Look at the link -- he's noting that @Alby commented suggesting that a non-custodial  option (without linking to another wallet) is coming to Alby.