I didn't read it yet, but the assumption is that the EU27 have a stake in a single financial market with the Brits. I don't think that's the case, because a financial sector feeds off the real economy, with genuine production and services. The City feeds itself very well with the EU27 economies, but Britain has little for the EU27 financial sector to feed off. I've read about suggestions that the main EU member states wanted to shrink the financial sector in the wake of the crisis anyway, but the UK prevented that.
It's not clear whether the EU27 sees Brexit as a chance to shrink the financial sector, or as a chance for Frankfurt, Paris and Amsterdam to grow their financial sectors, but no matter how closely politician0s and the financial sector are connected, there's really no reason to let the City profit from real economic activity on the continent.