If it was actually made by the Remington Custom Shop, it would more than likely have something like "Remington Custom Shop" engraved on the top of the barrel. This was extremely common for them to do, I have both rifles and bolt action pistols in my collection with this marking on the tops of the barrel. For rifles with a carbon fiber barrel, they marked it "Remington Custom Shop) elsewhere (I'd have to check mine). Unless the rifle is actually marked this way and/or has documentation proving it's provenance, I'd say that this rifle is just a generic rifle build, and not something that was made by the Remington Custom Shop.
You're completely correct, and unfortunately every rifle with a McMillan HTG stock is always an "M40A1," no matter how incredibly wrong it actually is. Same thing with all rifles that are built on a McMillan A4 stock, those are suddenly M40A3's or M40A5's. I can't tell you guys how many "XM's" I've seen over the years, just because the rifles have a McMillan A1-3 stock and a shorter barrel. Just because 1 part of the rifle is sort of correct (even though it's the wrong color and has other issues), idiots immediately call it an M40xx or whatever other military rifle it might look similar to. It's disgusting, and I see it on the forums all the time too.
This happens with all sorts of other rifles. Over on ar15.com it's a major problem with people posting some garbage AR15 build that has a Daniel Defense handguard in the Mk18 and M4A1 Block II clone threads. Literally everything about the AR15 will be completely wrong, but since it has DD FDE handguard, the owner think that it's a Block II clone. They clog up really helpful clone threads with trash, and then get absolutely butt hurt when people call out their trash.
Some of these guys are completely clueless, others are trolling, and the worst are the people who are complete assholes who fake everything and purposefully lie to customers. For example, take a look at the trash that Rocke Guns has listed for sale on Gunbroker. That asshole just makes whatever he feels like making, engraves completely fake markings, and them tries to pass the guns off as super rare contract overruns or whatever. How GB still allows him to sell stuff on their auction site is beyond ridiculous. He lies about everything, and the buyer thinks they bought a real military/police/whatever firearm. The buyer will tell all of their friends that he just got a genuine item, and just can't internally process the fact that they bought a fake gun.
As for the above rifle that's in question, it sounds like the guy selling it knows what he's doing, and it's wrong. He obviously put an M40A1 manual with the rifle, and even of the Remington Custom Shop had built the gun, they wouldn't be supplying M40A1 manuals. He put "M40A1" on the price tag that's attached to the rifle, and he's been calling the gun an "M40A1." He's calling it a Remington Custom Shop build even though the rifle isn't marked and there's no proof of it being built there. The guy sounds like a typical scammer. I wouldn't doubt it if he's been told by multiple people that this isn't even close to being qn M40A1 sniper rifle, and he just stubbornly claims something stupid like "how do you know it's not an M40A1, have you seen all of them that were made?" Also, that's not a smear stock on that rifle, it's just a regular HTG stock with the wrong camo pattern and incorrect grip/forend molded-in texture. There's literally nothing correct about that rifle in regards to it being an A1 clone, and it probably wasn't built by the Remington Custom Shop. The entire rifle is one big lie, and the seller should be ashamed of himself. And if the seller were an honest man, 5 seconds after a Google search he would know that this isn't any type of M40xx clone build. He either knows the truth and wants to lie to customers, or he's willfully ignorant.
@ChiefSillyHorse, sorry about the negativity, none of this is directed at you, this falls squarely onto the shoulders of the seller. I just can't stand shit like this, and sellers like this need to be called out on this nonsense.