It is impossible to “un-baptize” yourself, by the way.
Baptism imprints a special character on the soul. It cannot be “undone.” It may be possible to have your name removed from the registration rolls, but one’s name appearing on the membership rolls does not automatically mean that person was baptized in the Catholic Church. Converts to Catholicism who have already been baptized properly in some other congregation are not “re-baptized.” Their names are added to the membership roll when they convert. I assume that is done at Confirmation. The name appearing on the membership rolls is not in and of itself proof that someone was baptized in the Catholic Church. The website is sending a false message in advertising itself using the term “de-baptize.” There is no such thing as de-baptism, nor could there be such a thing.
As for those who leave, Father Z says “good riddance” while suggesting it’s probably uncharitable to say that. That’s not quite how I feel about it. My preference is not that they leave but that they repent.