I think they dealt with the interface with LN 9.0 and above. At-least to some extent.
LN was structured around the concept of "documents", which were binary objects (IIRC) passed around between the client and server and were the lowest abstraction layer. Everything in an LN app is based on creation, editing, transmittal, gathering, processing, etc of documents.
I don't know why such a thing was not evolved into a REST API sort of structure, but because of this concept of documents, and the state sharing mechanism between the client and server, we could program the system as if it was a monolith. I don't really know why the client was bloated, but that was a common feature of all IBM software (looking at you Rational Software Suite).
I guess they also could not evolve the software to make it more lean and multi-device friendly. Because it laid so much emphasis on security of identity, transferring the client to a mobile was impossible.
Domino server (LN server backend software) did allow for access from browsers (IIRC it required a plugin in the browser to handle the secure ID part of the client) but then, programming web enabled apps on the server was a huge pain. No clear support for templates, etc. It could have been evolved into a Django sort of framework but that was not done.
In my view, it was a lost opportunity. Imagine the amount of money IBM could have made if they could evolve LN into a Django Type web framework (where models could have been the documents).