The ROSCO didn't keep it a secret, TPE and the DfT knew but couldn't do anything about it. This is an effect of privatisation.
It wasn't formally announced initially though. DfT, TPE and the local PTEs were made aware of the issue months in advance any announcement. In fact the way it first got out was Modern Railways got hold of the information and then a few weeks later BBC North West Tonight did a report on it, followed by an interview with Tony Miles from Modern Railways on the topic. Then much later on Chiltern confirmed they've agreed a deal with Porterbrook for the lease of 9 x 170s.
So what's happened this time - Modern Railways have got hold of some information that indicates Porterbrook will not be leasing 323s to Northern for much longer. I'd expect we'll hear more over the coming weeks/months.
Let's bring this back to LM and Northern, ignore TPE... The LM franchise as it stands finishes at a similar time to Northern. So LM can't do a deal like Chiltern did above.
At present Govia will be in talks with DfT over an LM Direct Award which may include changes to rolling stock. Northern Rail bidders can't formally agree deals because they don't know who if they will win the next franchise yet. It's not uncommon for a deal agreed with DfT and an operator to involve replacing newer carriages with older carriages if it also includes an increase in the number of carriages.
DfT have put some conditions on DMU cascades, like preventing TPE bidders getting the units released by FGW and Scotrail and also preventing Northern bidders taking on DMUs which aren't due to be released. However, for EMUs they are being a lot more flexible with EMUs being in plentiful supply.
One poster in particular is a favourite of mine with previous suggestions of 377s coming to Northern. Now that idea died a death

Are you referring to my suggestion that there would be more logic in sending the 377/5s to Northern than ordering a huge load of Electrostars for down South, just to ensure the 319s are released? Some of the 377/5s could have been shortened to 3 car to increase flexibility up North. For example, having a 6 car 377 with corridor connections stopping at Salford Crescent instead of changing the infrastructure at Salford Crescent to cope with 8 car 319s, when £12m was recently spent to allow it cope with 6 car 185s (or equivalent), with no future proofing for 8 x 20m carriages.) The removed carriages could have gone in to other Electrostars, so no wastage.
Of course with DfT's London bias and ability to think logically not being mentioned on the job specification for DfT roles, that wasn't going to happen.