On a forum no-one can prove or disprove a claim like that.
They may well be a millionaire on paper. ie: Asset rich/Cash poor -> living off asset loans/credit cards.
They may well run a business that has a turnover of a £million a year and be worth a few million if sold, but after overheads/expenses may only draw the equivalent of the UK's average wage @ £26k p.a.
They may have won a lottery prize. "Self made" may mean going down to the newsagent themselves and buying the winning ticket.
IMO though, if they feel they have to make this unequivocal statement, so others look upon them more favourably, then they are only kidding themselves.