I recognise that is the case but the puzzling aspect is, since TOCs don't share Advance ticket revenue with other TOCs on the line of route (except where the ticket includes a connecting journey on another TOC), surely LNER has a strong financial incentive to ensure systems offer such fares?
The way the TOCs see it is that they show the fastest itineraries (slower ones are often only shown if they are a direct train with no changes) and then the relevant fare(s) for the itineraries are shown.
They are not interested in showing slower itineraries that involve stepping back; if the customer isn't happy with the price then the TOCs know that most people will simply pay the extra for a more expensive ticket.
Some people (a minority) will choose to either not travel at all or take another mode of transport; companies like LNER know that they do not have enough capacity to accommodate everyone who would travel if the fares were great value, so they rely on putting at least some passengers off.
The forum's site operates in a completely different way; the suppliers and the people who work for those suppliers are people who actually travel by train themselves and have no vested interest in maximising TOC revenue or putting some prospective passengers off rail or anything like that; on the contrary they want to encourage people to travel by rail and offer the best possible price, including offering itineraries which are 'overtaken' and/or involve 'stepping back', as well as combinations of tickets and all sorts of other stuff the TOCs aren't interested in providing.
It costs a lot of money to provide the level of service the forum site provides, however it won't cost
the customer more than booking the
same journey with the TOC and will often be cheaper!
Some TOCs aren't happy and accuse us of "distorting the market" (!) by offering lower prices, as well as placing loads of adverts trying to get as many people as possible to "book direct" and numerous other tricks to deter people booking with us.
People on this forum are made aware, through various threads, of the key differences in the approach and can make an informed choice of who to purchase tickets from.
If LNER wanted to offer the same service to customers they could easily do so, by switching to the same supplier (FastJP). But they choose not to, for the reasons stated above.