I think they're proposing switching your furnace from hard wired to plugged, so you have the option to power it either from an outlet (grid) or solar/generator/etc. This can't backfeed the grid, and is very safe.

It looks like when it's against code it's "by default": if an appliance is not explicitly listed as acceptable for a plug it has to be hard wired. ND and OK have passed state level amendments allowing it, some places have done this locally, and in many places building inspectors allow it with their discretion.

Yeah, maybe I misread. Switching it manually via a plug such that it could only ever be powered that way is fine (well, "fine" insofar as it alleviates my concern). I thought the suggestion was to add the new plug in addition to the permanently-wired solution.

On a beast of a HVAC system, a simple 15A wall plug isn't going to cut it for powering the whole thing, but if the furnace runs off e.g. natural gas and you just want to power the blower off a 15A plug, that solves the problem of switching power sources.