Not an iOS developer but I wonder if using the default browser requires using a different intent than the former behavior of "always open safari" did. Books, to me, seems like one of the more neglected first-party apps so it honestly wouldn't surprise me if this is just something that's sitting in the P4 column.

Only reason I assume this is all the Google apps on my phone bring up a sheet when I tap a link asking which browser I want to use-- with "system default" being one of the options.

Really there's no excuse for this, it can't be too difficult to adapt system apps to fall in line with the expectations for third-party apps.

to be fair, there is nothing other than safari on iOS...despite the window dressing.

Right, but other browsers have better behaviors to some people. For example, Safari's behavior of opening links as new tabs in the foreground is very annoying to me.