Portable Ops is one of the exaggerations of the legend of Big Boss.
It's been stated in canon that many of the exploits of Big Boss were heavily inflated or are flat out Blatant Lies. The general plot (Snake formed FOXHOUND, rescued Gray Fox (and possible recruited Python) and defeated Gene and the FOX unit) is canon, but the more ridiculous elements (such as the ICBMG itself, Sokolov's survival of Metal Gear Solid 3: Snake Eater and some of the recruitable characters, such as Zero, Ocelot and especially Gene and Cunningham) are not.
The FOX unit brought a large shipment of weapons with them.
Soviet soldiers are seen carrying M1911A1s as sidearms. Their supply of weapons must have broken down except their AK47s. The FOX unit must have brought a resupply of weapons to keep them armed.
