Good point, I hadn't noticed that in the email since the attached invitation says no such thing. Note, though, I only referred to when the certificate is received - which varies in time now a bit.
The oath ceremony letter we have does not say this, it just says you need a hard copy (of the paper one or the e-certificate) to apply for passport. But the email does after all say that - mea culpa.
I know I've seen some posts here from those who've applied for passport same day or day after (not the two business days) and not had problems. Would be great to hear if anyone has experience of passport office not accepting an application in these circumstances.
But since the email does say that (all the time? I dunno), I suppose they can or may have to delay. Quite possibly it's a cover-your-butt caveat from the government (after all, it only says 'minimum' two days) - or an instruction that's no longer relevant and they've never removed it. (And if someone has a certificate - why would they say you can't apply? They can take the application and check the system later - certificate # should be all that's needed for that).
My guess is that this is a 'when the system database updates' issue and that it no longer (always? only in some cases? perhaps only for in-person ceremonies? guessing) takes two business days for this to happen - with modern computer systems this should be, I don't know, maximum of a couple of hours, and even that only for database syncing (if their systems are now 'modern', of course).
With e-certificates - these could/should be linked to happen at same time anyway (make the e-certificate available for download only after recorded in system). I can see why there could be a (minor) issue or delay for paper certificates issued at an in-person ceremony i.e. they'd have to be printed in advance and probably not finalized / 'registered' in the system until sometime after the ceremony is over. (I presume that's why the up-to-five-days to send e-certificate after an in-person ceremony).
