Yes, that's probably how City thinks but it's a big gamble. The good old rule for business that it is three times as hard to win new customers as it is to keep old ones still apply. Before Brexit they had almost a monopoly in the biggest single market in the world. They are giving that up to be better positioned to grad market shares elsewhere at a time when the global economy is in the biggest bubble in history. Location matters of course but not nearly as much as it used to be now that we have internet and especially not after Covid. Also, I'm not sure how much a deregulated financial market and the domestic economy influence each other but it's worth keeping in mind that UK's national economy is rather weak.
It may work but I wouldn't bet on it. Only time will show.
The city firms, though, are already multinational organisations. They're in competition with other firms and organisations, not cities or countries. Whether business is handled by Barclays London office or the Frankfurt office doesn't matter to Barclays -- it matters to the offices concerned, and the people who work in them, but it doesn't matter to customers or shareholders, particularly.
So for banking and financial services, which are multinational anyway, it's a chance to grow new business.
Barclays (or Deutsche Bank or whoever) London can develop new business with Asia and the Pacific, particularly when British National (Overseas) citizens of Hong Kong start to relocate here in large numbers, that it couldn't have done inside the EU, and their EU offices can now introduce existing EU customers to new Asian and Pacific products and services more or less directly.
That's the theory, I think.
Anyway, we'll see what happens, but I'm certain that if either the City or the international financial markets were dissatisfied with the outcome, we'd know about it by now, and the pound would have crashed again, as it did when the result of the referendum was known.
We'll see, though, as you say.
