If it was valid, do you really think people would be talking about it being a problem here? Please use your head a little.
Also, two entitely different meanings of the word signing being used here. Signing as in signing a bill vs. Cryptographic signing. Adobe has some weird "halfway" thing that's more than painting the sig on the image, but isn't gpg.
Hooray for proprietary shit becoming accepted for legal use! Yuck.