Yes, without it, the extortionists will only get what you've left on your decoy wallet. It's a small price to pay for the plausible deniability actually. But it's better to place your seed in a box or a drawer with a lock or a padlock, so they can't believe you are giving them an old or a secondary seed, without the main part of your funds. They must think you are giving them the precious one. Otherwise they could try to threat you more just to be sure you are not deceiving them.