You did nothing wrong & the seed are not of poor quality.
Its a charistic of some seed & plain old physics just go ahead & remove seed coat or just let the cotyledons expand & the seed coat will fall off .....

Not deep enough can contribute to it but isn't the cause. Too dry soil can also contribute. Mostly, it just happens.

In most cases they will fall off on their own. Trying to pull it off usually damages the cotyledons. Some spritz them with water to soften them up. But what works best is saliva - spit - on your fingers and dab it one. The enzymes in saliva dissolve it.

Dave

Thanks!

For me it occasionally happens because the dry ambient air causes some of the tomato and pepper seed shells to dry and harden before they have a chance to fall off. They annoy me so I remove them. Don't pull or tug them. Carefully using a small needle-nose pliers to gently squeeze the shell on the ends causes them to expand like a clam shell and pop off. Easy to do although not absolutely necessary, but I like to give the babies their best chance at success.