Does this NM cable require protection?
They are apparently considering the attic accessible although it is not readily accessible. Most furnaces in basements have NM cable protected with EMT conduit and that is probably where they are equating that situation to this one. However, in a normal basement the cable would be readily accessible, meaning you can walk right up to it without any tools or ladders.
So, it doesn't need protection since it is not readily accessible being that it is in an attic that needs a ladder for access. However, it does need to be secured and supported according to the code. Meaning within 12&quot; of of the switch box in your picture and every 4 1/2 feet thereafter.
From the 2014 National Electrical Code:

334.30 Securing and Supporting.
Nonmetallic-sheathed cable shall be supported and secured by staples, cable ties, straps, hangers, or similar fittings designed and installed so as not to damage the cable, at intervals not exceeding 1.4 m (41⁄2 ft) and within 300 mm (12 in.) of every outlet box, junction box, cabinet, or fitting. Flat cables shall not be stapled on edge.

It is up to the HVAC technician and electrician to agree on a method and location of securing it to the AC cabinet.
Good luck!