Is the ice cream testing via the Inverted test pyramid an anti pattern?

Yes and No

More often than not, an inverted pyramid (ice cream testing) is an anti-pattern, but there are circumstances where it is not. Your example of needing to rebuild an API in a different language is one such example. Some other circumstances where you might want to invert the pyramid include: You have an integration to a third party API. Your lowest level tests are really there to ensure that the API hasn't changed on you. Most of your tests are going to be end-to-end to make sure your application correctly formats data for the API and correctly handles responses from the API.

You have an integration to a specialized physical device. The same basic principles apply, except that a lot of the end-to-end testing is likely to be manual because specialized devices (like turnstiles, RFID-managed lockers, etc.) are not good candidates for automation and simulators will not necessarily give completely accurate responses.

You are testing communication between different applications/systems/apis. If your primary focus is the communication between the different systems, then the pyramid will be inverted. You are working with legacy code. If you are working with legacy code, there may not be a choice in how you automate. Any stable legacy system is likely to have UI and processing logic tightly intertwined, making unit testing a challenging exercise if not impossible.