He can claim it all he wants, but its a load of crap
Sounds like Seattle and Sheppard's agent had already assumed he wouldn't get drafted and worked out the details of his UDFA agreement before the draft was finished, but didn't actually sign it until the last name was called and they were certain. Sheppard probably didn't realize or didn't bother to specify that the actual signing didn't occur until the draft was over.
I'm speculating totally, of course, but it's the most plausible explanation to me. It seems really silly that an organization like the Seahawks would be ignorant enough to do something that stupid for no reason. I mean, it's not like there was some huge race to get him signed before all those other teams that were so desperate to get him that they'd just passed on him 7 times.