First of all, in game flavor text descriptions do not necessarily correspond to game mechanics. I've seen people make this mistake about 8,000 times in probably a minimum of 30 different games.
Secondly, in this case it is true.
Armor provides the full value of defense against mass drivers, but the square root of that value against beams and missiles. So for instance if I have 16 armor on my ship, I have 16 defense against mass drivers, OR 4 against beams, OR 4 against missiles.
This is referred to as off-type defense, and it works the same way for shields vs. missiles/mass drivers and point defense vs. beams/mass drivers.
This is somewhat complicated by the fact that defense is not a static value; it rolls from 0 to max, is depleted during each round, and is replenished at the beginning of every round. There's also the fact that the square root is apparently taken after the roll, so having 16 is not as ideal as it would seem to be given that your average roll would be 8, which is one shy of 9 (which would give you 3), so it will only give you 2. I have been intending to verify this but haven't done so yet. Additionally, the off-type defense is depleted from its base value, not its square rooted value. While this would seem to make off-type defenses too powerful, it's worth noting that even a depletion of 1 from the base value can lead to a reduction in the square root value. However, in general this means 1-3 attack ships will not fare well even against off-type defense.
One thing to note is that it's not really adding defense-it's an either or thing. If you're fired on by a beam weapon and you have 16 armor, your armor is treated as 4 shields (for simplicity's sake ignoring the complexities above). Assuming this is depleted somewhat (let's say by 1) and you are fired on by a mass driver weapon in the same round, you are now rolling from 0 to 15 instead of from 0 to 16 for your defense against mass drivers.
I hope that makes sense.