They probably do that in parallel, the point of the visitor disclosure isn't to reveal the social media history, it is so even if the social media history itsel

They probably do that in parallel, the point of the visitor disclosure isn't to reveal the social media history, it is so even if the social media history itself, which they have through technical means, doesn't reveal any basis for exclusion, the failure to disclose parts of it (which comparing the disclosure to the records they already have identifies) can do so, as well as pointing authorities to what the visitor thinks needs hidden and should be investigated further.