I wouldn't delve into the lore too much, otherwise you'll see holes like that.

The shuriken weapons are a good example of this. According to the fluff, shuriken weapons despite being so advanced that it can turn a solid black of matter into thousands of nano-thin shurikens, lack the range and accuracy of bolters as they're essentially firing flat discs with very little to stabilise the flight, whilst bolters fire spin stabilised slugs from rifled bores.

So you have a 1,000,000,000,000 year old race than can build weapons that literally manufacture its own ammo getting outgunned by a bunch of newcomers using something as primitive as cylindrical ammo in rifled barrels...

Because autocannons aren't tank cannons. You're not looking at an M1 Abrams, you're looking at a M2 Bradley