LGPL 2.1 6c - on request, you provide the user with object code they can link against their custom version. Should be enough, if no other terms of the app store get in the way?

(VLC is an example of a well-known program that relicensed to LGPL and now is available in the app store, so clearly it is possible)

And how does the user then get their relinked app onto their phone?

(There are plenty of App Store apps with LGPL code in them. The question is, should they be there? And VLC is not a good example, since presumably, I don't know - but I think all of it is LGPL or in any case, open source. The user can rebuild all of VLC in that example. The situation with Boden is going to be that a an app, probably proprietary, has a Boden library in it.)

Isn't the problem of "how do I get the app onto my phone" the same in both cases?

If the app is open source (or at least the source code is accessible), the user would proceed to the source repo, clone it, open the Xcode project and relink to the framework version of their choice.

Others have pointed out how it could work with closed source apps. It's a bit more complicated, but it certainly is possible.

It would be prudent to advertise this expectation of the app developer hoping to use the LGPL version. (Whereas with for instance the MPL and other licenses, no such need exists.)