Why would it be assumed to be plastic boxes when it was specked to be metal boxes?
If metal boxes were specified, then metal boxes should have been included in the bid pricing unless it was negotiated during bid time.
Conduit/mc was spec'd also. But NM was approved as a substitute.
Any rational mind would probably understand that metal boxes go hand in hand with conduit/mc and non-metallic boxes go hand in hand with non-metallic cable
From Allied Moulded website - 1098 single gang box:
"Single gang electrical box for use with nonmetallic sheathed cable"
Granted, it also says they're for residential use - which is another instance of various parameters going hand in hand:
residential, NM cable, nm boxes
commercial, EMT/ MC, metallic boxes
2 of the 3 parameters are already out of the perceived norm. The third would be assumed in my mind
Like I wrote on page 1, a lot of the stuff in the general notes on a set of prints is nothing but stock filler jargon that means absolutely nothing to the engineer or architect.
Somebody just didn't find it necessary to go into every trivial detail when NM cable got approved as a substitute, they probably gave no thought at all to the boxes.