Can I run NM wire through this type of flex legally
According to the 2015-16 UL White Book, this product is only allowed to contain the following:


Optical fiber cables (Types: OFNP,OFNR,OFN,OFNG,OFCP,OFCR,OFCR,OFC,OFCG).
Communications cable (Types: CMP,CMR,CM,CMG,Cross-connect wire).
Power-limited fire-alarm cable (Types: FPLP,FPLR,FPL).
Community antenna television cable (Types: CATVP, CATVR,CATV).
Low-power network-powered broadband communications cable (Types: BLP,BLR,BL).


Which means no NM cable, no lamp cords, no extension cords, nothing but the types of cable listed in the table above.


  2013 UL White Book
  
  Cable Routing Assemblies (QBAA)
  
  Use and Installation
  
  This category covers routing assemblies for installation of conductive
  and nonconductive optical fiber cable, communications cable/wire, powerlimited
  fire-alarm cable, community antenna television cable and lowpower
  network-powered broadband communications cable. Cable-routing
  assemblies are intended to be installed in accordance with ANSI/NFPA 70,
  "National Electrical Code" (NEC). Cable-routing assemblies are only suitable
  for the installation of cable/wire noted in the following information.
  Individual routing assembly systems differ in their construction and,
  therefore, their components are not interchangeable with other routing
  assemblies or fittings of other systems. This category includes pliable
  lengths, rigid straight sections, elbows, bends, and fittings such as expansion
  joints, female and male adapters, and couplings.
  These products may or may not incorporate end fixtures or covers.
  
  


NOTES:
- I had trouble finding UL marking on this product, so this information could be inaccurate. Check the product/packaging for an official UL mark and listing. Specifically look for the wording "General-use Cable Routing Assembly" and/or "ANSI/UL 2024"