I think it was more to do with connecting accounts where spending occurs.
Let’s take credit cards as an example where users currently do plenty of their spending due to various offers (cash back, points, travel miles etc). Monzo doesn’t offer something like this but ultimately Monzo users are likely to want to track this against their set budgets.
OP mentioned other banks but this were simply examples of external accounts. I suspect it is geared more towards where spending occurs outside of Monzo and where savings occur (Nutmeg or Hargreaves Lansdowne). Monzo will never offer these most likely so will not lure those users however existing users and new users are likely to have these and so it would be a great feature to be able toa analyse all these in one place under Monzo.