The website mentions the expansion of T5B and T5C to the north and south and a reconfiguration of space in T5A to increase capacity. However this isn’t mentioned in the consultation document itself or in the timeline.
I’m speculating that this involves building out B&C to the extent originally envisaged and finding reconfiguring office space BA have vacated in T5A, as has mentioned in other threads. Maybe the unused lounge space in T5C might even be put into use?
What is clear from the timeline is that the airport wants flight numbers to grow before the third runway opens and that serious terminal capacity won’t follow until after the the runway opens. So in the medium term pressure on all the current terminals will increased with this plan.
I think it is a shame that the transit (presumably airside) between all the terminals except T4 that was floated in previous plans has been dropped. So even if T5/T5X/T5XN becomes a hub for one alliance a transfer from say T5C to T5XN will remain fiddly and probably involve buses.