You're a bit off. Looking at the corrected 6th printing of Total War:

Page 216, Cluster(Ballistic) = Damage/10+1, so the base damage is 2.

(Absolutely nothing at all gets Damage/5+1, I'm really wondering where that came from.)

Page 215, Non-conventional infantry attacks vs mechinf get doubled, so 2*2=4.

Back to page 216, attacks against conventional infantry in clear hexes is doubled, so now we're at 4*2=8.

The notation on page 217 about doubling damage to mechinf is simply restating the earlier part on page 215, so nothing happens.

That's it, the end total is 8 damage. That's still a fair chunk so you're advised to keep mechinf away from cluster guns(and especially big missile racks) if appreciate cover isn't available, but it's hardly a case of one hit wiping out the whole platoon.