The guy in question knows what he's doing, so he must've had a reason that isn't apparent from the outside looking in. For instance, maybe they just stumbled on an unusually good deal on a bunch of those 24-bit converters on eBay.
In low-to-medium volume production with aggressive cost targets, part selection is as much an input to engineering as it is an output.
There could be purely technical reasons as well. Antialiasing requirements are somewhat relaxed with sigma-delta converters since they oversample by nature, and that's a win because filters are always a hassle. Especially if you're designing a spectrum analyzer, where you'd like to achieve spur rejection in the vicinity of 100 dBc or better. So either he spends a lot of money buying ready-made LPFs from Mini-Circuits or whoever, or he spends a lot of board space (likely with accompanying shielding requirements) on discrete inductors and capacitors. Unintuitive choices can make more sense when you're simultaneously fighting for space, power, and cost like they are.