Necessity of electrical boxes
Electrical boxes are required by code for some wiring methods.  300.15 specifically mentions conduit, tubing, Types AC, MC, MI, nonmetallic sheathed cable, and other cables. However, 300.15(H) says that insulated devices do not require a box, as permitted by 334.40(B).

300.15(H) Insulated Devices. As permitted in 334.40(B), a box or conduit body shall not be required for insulated devices supplied by nonmetallic-sheathed cable
334.40(B) Devices of Insulating Material. Self-contained switches, self-contained receptacles, and nonmetallic-sheathed cable interconnector devices of insulating material that are listed shall be permitted to be used without boxes in exposed cable wiring and for repair wiring in existing buildings where the cable is concealed. Openings in such devices shall form a close fit around the outer covering of the cable, and the device shall fully enclose the part of the cable from which any part of the covering has been re-moved. Where connections to conductors are by binding-screw terminals, there shall be available as many terminals as conductors.

There's also 300.15(E), which allows devices with an enclosure integrated into the device.

(E) Integral Enclosure. A wiring device with integral enclosure identified for the use, having brackets to securely fasten the device to walls or ceilings of conventional on-site frame construction, for use with nonmetallic-sheathed cable, shall be permitted in lieu of a box or conduit body.

So, yes. If a receptacle was designed and listed specifically to be used without a box, then it can be used without a box.