I find this claim pretty ridiculous. A lot of mecha are humanoid, so it's only natural that their hand-held armament be of human origin in design. These include knives, swords, pistols, smg's, rifles, sniper rifles, shields, etc. This is like me saying Macross Plus totally rips off Mobile Suit Gundam because they gave the VF-11, YF-19, and YF-21 shields, or anyone who wields an oversized sword totally stole the design from Cloud in Final Fantasy VII (who obviously isn't the first dude to wield a big ass sword)
While not plentiful, I can only think of two shows before Gundam 00 where mecha used sniper rifles; Evangelion with the positron rifle and Gundam 08th MS team's GM sniper.