Your source is wrong. The T-34's armor was thin but extremely well-sloped that more than doubled the effectiveness of its armor plate. The M10 had like the T-34 only 45mm of armor, but sloped unfavorably. It was a stripped down M4. But at no point was the T-34 designed to sacrifice protection for speed.

The M18 was a deadly weapon in anti-tank combat. That its armor was thin was almost irrelevant. In a successful deployment of TDs, a panzer column was often destroyed in a matter of seconds without firing a shot or scoring a hit in return.

As TD crews and their combat records showed time and time again, it was the German infantry AT weapons that posed the greatest threat to TDs, not the cats.