The latter.

Be warned that each tranche of miles transferred in from non-flying partners will expire 2 years after it reaches your account. As a temporary COVID measure, any miles that otherwise should have expired on dates prior to December 2022 were not removed from users' accounts. Now that December 2022 is in the rear view mirror, this exception no longer applies. All miles that should have expired in each month up to and including December 2022 can now be removed, without additional warning/notice, from user's accounts. You do not, as you suggest above, get a "brand new" full period of 2 years starting now, on top of the 2 years plus the Covid exception period you already received. Your miles were already living on borrowed time; that time is now up.

The majority of FB users would consider amassing a rather large stock of miles to be a bad idea; to run a FB account solely to receive transfers from non-flying partners (those points would "live forever" in the original Citi/Amex/Chase accounts if left there, and you have many other options to use them) and to never actually "use" those miles (Why transfer into FB when you don't use FB miles? What good is a "transfer bonus" if that just means you end up losing an even greater of Flying Blue miles, on top of the Citi/Chase/Amex points you wasted? Don't you see that you're falling victim to the marketing ploy that triggers "kaching" signs in users' eyes when they see a "transfer bonus" or "transfer deal" and fall for it? A deal is only a good deal if you benefit from it - and yet here you are, looking down the barrel of losing 280,000 FB miles without anything to show for all that credit card spend) is an even worse one.

You need to either make a booking using those miles immediately (and no; if you book something speculatively, and later cancel that reward flight, you will lose those miles; you cannot extend the validity of miles by making, and then cancelling, reward flight bookings).

In short, if having a Flying Blue account is actually of benefit to you, you really should factor in flying at least one XP-earning flight on Delta or any of the Skyteam or Flying Blue partner airlines on as infrequent a basis as is necessary to extend the lifetime of your entire mileage balance.