You misunderstand. Those sources are referring to AOR spacing, not spacing of individual applications.

No one inquiry hurts you. But some banks looks back only a certain number of months (Citi, for example, 6) at inquiries. So the whole reason for spacing out AORs (where you apply for a bunch of cards "at once", then pause) is to have the cards from 3 cycles ago be more than 6 months ago, and thus "invisible" to those banks that only care about inquiries in the past 6 months. (Though you have to do a bit more than 91 days, since 2x91 days is not quite 6 months always.)

Exactly how long ago a single inquiry was doesn't matter to anyone. It's whether it's within the period of time the bank counts or not that matters. No bank counts a 45 day-ago inquiry any different than a 90-days ago inquiry.

Furthermore, all that hurts is too many inquires on the same bureau as your new card pulls. But Barclay always pulls TU, and hardly anyone else (including Citi) ever pulls TU. So your Barclay pulls are "invisible" to most other banks most of the time.

So for all these reasons, the US Barclay application shouldn't factor in your Citi application timing at all. It's applications from other banks (even if they're longer ago, but less than 6 months) which are all that Citi will count.

As you long you have good (and sufficiently long) credit history, you should be fine, no matter when you apply for your first Citi card. (Just be aware of the timing rules for applying for a second or third Citi card, since those intra-Citi timing rules are much more important than this supposed 91-day rule you mentioed.)