That reads like the typical lawyer speak you'd get from any firearms company.
I check my own head-spacing at home with a set of go/no go gauges. If that weren't just a bunch of @$$ covering lawyer speak, then why would Mossberg sell surplus "mil-spec" AR-15 barrels by themselves on the aftermarket? Every firearms manual I've ever read tells me that my gun should never be broken down any further than a basic field strip unless performed by a qualified "brand X" service technician of qualified gunsmith. That being said, I've torn apart and rebuilt just about every firearm I own and I've still got all my eyes, fingers, and toes. I'm not sure what you're seeing that I'm not, but everything that I've read about the entire MMR line tells me that any and all parts are interchangeable with the vast array of aftermarket AR-15 parts out there. I know you're just trying to be thorough, but you're looking for a problem that simply isn't there...