The ESRB thing is likely either (PR spin) a convenient excuse for a team that wants to maintain relations with civil aviation manufacturers and avoid any association with combat, or (misunderstanding) someone making an assumption about how the ESRB rates things.
The ESRB rates the base game, and if add-ons aren't included, they don't affect the base game rating. (There's even a warning that add-ons can change a game's content from the original rating.) If an add-on had non-firing weapons on a realistic representation of an aircraft, I would be shocked if it affected a game's rating. I've had communications with the ESRB since it was founded, when I was a game magazine journalist in the 90s, and have worked with them on and off since on various issues, so this is an informed take, but I'll caveat it that I've never worked with them on anything MSFS-related.
EA/Jane's USAF had an "E for everyone" rating. Just sayin'.