There's a very simple (implicit) rule that is very much put on a pedestal in D&D, especially in 5e:
##If You [the GM] don't like it, Change It!
If You [the GM] don't like it, Change It!
What does that mean? Well...
If the GM feels that their orc tribe is equipped with Longbows and has a +6 for that, this is their decision and it is not in the right of the players to challenge this decision because the GM is the world. 5e makes it a big part that the GM has every right to change the world as they deem fit, to invent monsters and change them as they want.
For example page 5 of Xanathar's guide to everything:
One rule overrides all others: the DM is the final authority on how the rules work in play.
And in the Monster Manual itself there is this:
Feel free to tweak an existing creature to make it into something more useful for you.
In this vein, the GM did decide to change the Orcs to be something they want: Killersniperorcs. True, their challenge rating was obviously too high for your party, but it was their right to do so.
But the Players have no basis for their argument if it starts with "In the monster manual..." because D&D 5e is a game that practically yells at the GM "Invent your own stuff, we just give you a toybox".This lazy invent everything yourself attitude is what I hate in some games. And this very thing about 5e takes away the players very ability to challenge GM decisions on the basis of the monster-manual: The GM is the final and only arbitrator, especially when it comes to monster statistics as the 2 quotes above point out.
And they have to be able to do this tweaking because there is one big thing that designing monsters in D&D5e demands of the GM: Playtest it.
##TL;DR
TL;DR
As mean as it sounds: don't challenge the decision of the change in itself, ask the GM to explain how challenging they thought this should be. They might be still playtesting this specific variant of orc and adjusting it for further use!
