As someone whose played and grew up with Halo 3, Reach and 4, anyone who says Infinite has a shit weapon sandbox is wrong.
It is my favorite sandbox because I can ACTUALLY chose to use most weapons/they have a role they work well in. Whereas before a lot of weapons felt useless to bother with.
Also don't get me started on how Halo 4's loadout system just RUINED the sandbox. Complete destruction of what I love about Halo's weapons have distinct roles.
But they might assume sandbox = how much weapons a game has. So let's go from that angle...Halo 5 beats them all, and Halo 3, Reach and Infinite are honestly similar amount of weapons. I've heard commenters say the opposite when literally presented evidence they are wrong. But alas, nostalgia is strong.