The John Thomas Pierce who married Ethel Turner can be identified wit her in 1911 and I'm sure is a different John Thomas.
I suspect you may have the correct certiificate,even though it is only in the name of John.The family is at Castle Terrace,Flint in 1891(ref RG12/4608/71/30).John has a brother Thomas,so it is possible it was his middle name.
Regards
Roger