Did you read the referenced Apple Developer article above? Obviously the Apple tech support person did not. It states for all iOS 18.7.3 security updates they are...
Available for: iPhone XS and later
That doesn't imply that this update won't be available for newer iPhones; in fact, just the opposite.
Furthermore, anyone with a Developer account for Public beta account can see and download iOS 18.7.3 on any iPhone that can run iOS 18. It's more likely that Apple hasn't released it to newer iPhones yet as a public release. iOS 18.7.2 dropped 3-4 days after the corresponding iOS 26 release.
Then again, Apple may decide not to release it all, but that would leave a lot of iPhones 11-16 with RC iOS 18.7.3. OS installations. A confusing state to be sure.