This is a good question! Let's take a look at a few rules which work together here:
<R9> is the rule which covers polycarbonate legality:
<R9> A limited amount of custom plastic is allowed. Robots may use non-shattering plastic from the following list; polycarbonate (Lexan), acetel monopolymer (Delrin), acetal copolymer (Acetron GP), POM (acetal), ABS, PEEK, PET, HDPE, LDPE, Nylon (all grades), Polypropylene, FEP; as cut from a single 12” x 24” sheet up to 0.070” thick.
However, <R5a> explicitly states that VEXpro products cannot be used.
<R5> Robots are built from the VEX EDR system. Robots may be built ONLY using official VEX EDR components, unless otherwise specifically noted within these rules. Teams are responsible for providing documentation proving a part’s legality in the event of a question. Examples of documentation include receipts, part numbers, official VEX websites, or other printed documentation.
a. Products from the VEXpro, VEX IQ, or VEX Robotics by HEXBUG product line cannot be used for Robot construction, unless specifically allowed by a clause of <R7> or cross-listed as part of the VEX EDR product line.
However, the bolded portion above does point us to R7 for a possible exception:
d. Any parts which are identical to legal VEX parts are permitted. For the purposes of this rule, products which are identical in all ways except for color are permissible. It is up to inspectors to determine whether a component is “identical” to an official VEX component.
The gray polycarbonate you refer to would fall under <R9>, because even though it happened to be purchased from VEXpro, it is functionally similar to any other vendor's legal polycarbonate, per <R5d>. Therefore, it is legal (provided no other rules are violated, such as the 12” x24” restriction in <R9>).