OK. So basically what this amounts to is:
1) almost doubling the time for initial priority boarding to pass through the scanners (by effectively halving the scanners from 2 to 1 during the first phase of priority boarding)... and it is noticeable, if you hang back and wait, and watch.
2) just about everyone* is eligible to board at once (granted using front and rear system) *EXCEPT the ONLY people held back are just rows 6-12 who are NOT high enough status holders, otherwise it's "all aboard"! (Rows 3-5 & 13-14 branded as "X" come with "priority boarding" 737-800).
If this is happening as a new standard, and not just when a flight is delayed, then I think it is a major dilution of the "priority boarding" perk for cabin class and status holders.