I'm not so sure about this.
Abortion was legal and widely available in the US at the time these supposed events took place. IMHO M would have chosen to terminate a pregnancy, she had her life plan, and an early marriage and a baby were not part of those plans.
As far as a 'sealed annulment' I've never heard of an annulment or a divorce being 'sealed', they're public records. Sometimes the financial and child arrangements aren't public records, but the final decree is. But I suppose it could be a possibility. Annulment is pretty specific as to the grounds; lack of consent, duress, prohibited marriage (bigamy, incest), or an underage marriage. M was 18 but I suppose her groom may not have been. This is all supposing it's true, which I doubt. Saying it was 'sealed' in the article would 'explain' why there was no record if someone went 'snooping' for it.
And the allegation that this child was never 'official', ie registered at birth is pretty suspect to me, too. It's pretty hard to do anything without a birth certificate in the US. And birth certificates are done in the hospital here. The nurse fills it out and the parent signs it. The hospital sends it off to the registrar. If this putative child exists and was born in a hospital, the birth would have been registered. And I highly doubt that an 18 year old primigravida in the US would give birth at home, especially back in the '90s. Home births are pretty rare in the US as it is. I put the 'not registered' in the same category as the 'sealed annulment'. A good reason why a 'snoop' wouldn't find anything.
On a scale of 1 - 10 with 1 being 'no way' and 10 being 'absolutely true', I'd give this a 3.