Some people might think that the secret identities are pointless after Oblivio and that there was no danger so that excuse was not valid anymore but, there was danger right there… It was just downplayed since the focus of the episode was elsewhere.
The dangerous part is what they showed in heroes day
The lovers that know each other identities:
-Alya got compromised by covering Nino when he was capable of protecting himself (Chat usually does this but it’s almost always pointless sacrifice like getting in the way of the attack instead of pushing the attacker to the side, Ladybug always finds a way to stay safe and help Chat because her judgement isn’t clouded by feelings)
-Nino got compromised because he was more focused on Alya’s loss than the mission
-Not to mention that any mind controlling akuma could get the identity of the partner
The hero whose identity is known by everyone:
-Chloe’s whole family got akumatized effectively getting her compromised
-She can never have her miraculous permanently ‘cuz Hawkmoth would target her while she’s defenseless like asleep or in the bathroom
Oblivio was not a mind control akuma but Ladybug kept her partner away from the fray because she wanted him to stay safe, almost said “Adrien” out loud when he was in danger and Chat Noir almost called her “Marinette” while they were just having a conversation. It was a dangerous situation and a slip of the tongue could have done a lot against them