They are real chips, but generally not scarce chips. Mega-dealer Jerry Wall had multi box quantities of many Nevada chips. I don't know whether Jerry did it, or whether BillA did it, but many of these chips were encased in clear plastic and displayed at Gamblers General Store when Bill was helping them dispose of Jerry's inventory. The quality of the encasement was (in my opinion) poor, with lots of air-bubbles trapped inside. There was usually something stamped on the bottom saying something like "Genuine Nevada Casino Chip" (which they were). The list of eBay auctions you linked to is a pretty good one to save for future reference because these chips are all pretty common, even though many like the Dunes golf-course inlays have attractive designs and are older chips.

I wondered what happened to all these 'slabbed' chips; I guess we now know [g].

There is another category of encapsulated chips... those made to give away by the casinos as awards or promotions. I have quite a number of these including Hyatt Tahoe and Sahara Tahoe. These are generally quality castings and are in unusual shapes, like hemispheres and dodecahedrons. These generally sell for from $15 to $50 and make nice display items for your 'what-not' shelf at home. See an example below.