legitimate
1, Conforming to the law or to rules.
"his claims to legitimate authority"
synonyms: legal, lawful, licit, legalized, authorized, permitted, permissible, allowable, allowed, admissible, recognized, sanctioned, approved, licensed, statutory, constitutional, within the law, going by the rules, above board, valid, honest, upright; More
2, Able to be defended with logic or justification; valid.
"a legitimate excuse for being late"
synonyms: valid, sound, admissible, acceptable, well founded, justifiable, reasonable, sensible, tenable, defensible, supportable, just, warrantable, fair, bona fide, proper, genuine, plausible, credible, believable, reliable, understandable, logical, rational
"these are legitimate grounds for unease"
Saqifa wasn't according to any of the above. Constitution (Shura) was out of the question because it wasn't a public gathering or event. And all parties weren't present and available. Vast majority didn't have a clue about this incident. Even the Shaykhain weren't aware about it. So how was this a public gathering or event?
Later on people started to become aware of what all of a sudden happened. Some started to accept or come to terms with it. Abu Bakr was a good man this, that and the other. It doesn't matter, it wasn't LEGITIMATE full stop.