Not at all true! The land registry might assume some responsibility for becoming the official definition of boundaries, but there can often be some complex issue about access rights or relations with neighbours that's recorded on the deeds, but doesn't fit into the land registry's data model and so might otherwise become lost. This is usually a loss for the property owner.
I currently spend lots of my evenings standing around with the neighbours staring at orange tape on sticks and worrying about just such a boundary issue, much of which would become a lot easier if only we could turn up a copy of the deeds from before the 1960s.