This has been 'discussed' on other forums (besides the vaunted CAS City); Mr. Cuts Crooked is the only person to ever make this claim that I've seen. Hawg is absolutely correct about the chemical nature of the material used in percussion caps - it's only impact sensitive, which means the force time history must have at least one sharp edge.
I'm not saying one should not use caution; that's prudent any time you're working with explosives, but I don't believe carefully applied thumb pressure has any likelihood of igniting a percussion cap. You can believe whatever you want.
And no, I'm not going to go to CAS City and start an argument - I don't really care one way or the other what the members of that forum believe on this subject.