Can Laird Connectivity's Flex PIFA be mounted on a metal surface?

No.  That said, by placing a 1.5 mm thick piece of polycarbonate between the metal surface and the antenna, the FlexPIFA can be used.
Detailed placement and recommendations can be found in the Datasheet:
2.4GHz FlexPIFA
Dual-Band FlexPIFA
FlexPIFA 6E

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