Well the P90 isn't all that far fetched. Look at the AK47, made in 1947 and still in use today, although not by any decent military. That's 62 years.
The M16 was designed in 1960, and variations of it are still in use today by many NATO forces. It's been around for almost 50 years and will probably be for many more.
The P90 made in 1990, and then shown in 2062. That's 72 years.
Not all that much longer than the ak47, which doesn't look like it's going to be dropped by anyone using it any time soon. And why not, GDI must be running low on supplies, look at the freaking city. I don't think cracking open an old crate of TD-era weapons for some security guards would be such a bad idea. A gun's a gun and no matter how far into the future you are, bullets still hurt