I'm fully aware that sheets can be made from a pressure chamber. The context of the conversation was "available before pneumatic craft".Hmm. Not to sound like a broken record, but you could also read the quest book. The quest "Beginning of the End" explicitly states you can use Pneumaticcraft plastic to make MFR plastic sheets.
*After* you go beyond the plastic and make a Printed Circuit Board, which is the 10 + 5 minute process with items decaying in the second part that is so ridiculous.And, unless I miss my guess, that quest also rewards a rubber tree sapling.
And of course, doing it that way means that MFR items such as conveyor belts that don't use plastic sheets and instead use rubber are not available until after that quest is done.