Which new rolling stock, I am presuming that it is a cascade of 377's, when the 377/6's come on stream?
376/6 are to provide 10 car services on some metro routes on Southern so no cascade there other than the 456s to SWT.
The twitterist is possibly putting several bits of information together.
1) Some of the Thameslink services will replace existing Southeastern services thus Southeastern will have some "spare" rolling stock to Strengthen existing services. (When Siemens eventually deliver the 700s)
2) 12 car services were meant to be introduced on Southeastern metro/suburban services after NR did the power upgrades (almost done?) but no one has heard anything on the subject of the additional rolling stock required to do this for a long time so they might be in line for some of the order that Southern is doing for the government (Not the 377/6 or 377/7 orders) where it is unclear who the final TOC who actually gets the rolling stock is (but plenty of rumours - everyone assuming their local line!). It may be the re-franchising delays have pushed this to the Government (via Southern) to sort the new units rather than the next franchisee as possibly originally planned.
3) TOCs may just be going for narrow 2+2 seating on new stock to maximise standing space rather than 3+2 and are spinning this as positive as there is no longer a middle seat!