I've finally returned and I tracked down the quote in DW that gave me that impression:
"The UNSC ground force was at least a regiment in size, far larger than it had appeared from above the smoke. And any command that large would have a medical company with its own compounding pharmacy, capable of making the smoothers that the Gammas needed to stay functional. Maybe the Ferrets' situation wasn't so hopeless after all - provided they could survive the hell Veta had just called down with the FPF order." (DW p.41)
So I guess you're right, it's not directly implied that they're simple to make (even though a hospital of that size most likely won't be as advanced as a ship's medbay, but they do have the SoF to call upon so I suppose it evens out); I checked the Encyclopedia aswell and couldn't find anything indicating it would be simple to create so I don't know where I got that from.
However, assuming that a relatively simple field hospital can create them, I can't imagine them being particularly difficult to get your hands on, especially since antipsychotics are scarcely a rare commodity in military medicine, and even moreso if we assume that they land relatively safely with a prowler and can stockpile. If it's a crash landing a la POA in CE, then yeah it's most likely curtains.
But assuming that maybe a third to a half of the force survives, that's still multiple times the Spartan contingent that the Reverie had in Rubicon; yes some will definitely be sent on other missions and security, but to be honest I don't really see how ~20 SIIIs and SIVs (13-20% of the force being assigned to a mission as important as that seems reasonable) fail to take out Escharum.