This is what is known as balancing the game so its never truly one sided. If you ever play something like inquisitor, a space marine has strength and toughness easily four/five times greater than regular characters.I find any space marine article where the few face the thousands since in game 20 ork boyz are enough to mug a 10 man tac squad.
Think about it, as alien as an ork, wearing heavy armour and wielding armour piercing weapons vs an ork with cobbled together gear and next to no armour. Seems pretty reasonable that in a straight fight, a ten man squad of marines can deal with a mob twice that size.
As for the thread, for me its something you tend to see more in fan made stuff. What really makes me roll my eyes or cringe would be when you see some captured loyalists escape the eye after years, or some traitors turning loyal, and then going on to join a chapter, or to even form their own chapter, like its no big deal.
I mean come on, new chapter requires a founding, which requires the sanction of the high lords; and at this time there is no doubt they would sooner destroy a potentially tainted marine than give him authority over a thousand more marines. At least they wouldn't even consider listening or trusting him until a number of stringent, likely fatal, purity tests.