This is Spring's "erring on the side of caution" trademark enforcement mechanism, not trademark law itself. I highly doubt that a court would acknowledge that Affinity has some trademark claim over all short strings of Greek letters.
Is it? Or is it a mechanism of preemptive enforcement put in place in response to past action by ACS or some generally applicable legal rule requiring preemptive presumptive enforcement of known trademarks on platforms, hence the reference to Spring’s “legal obligations to protect the assets of Affinity Client Services” in the letter from legal?