They talk about it in the article.

In theory, any phone with the right hardware for a carrier’s cellular frequencies should be able to use VoLTE, and any capable phone with a supporting plan should get VoWiFi. The reality, however, is more complex. Many carriers only permit VoLTE and VoWiFi on devices they sell or have officially tested. This means that even if your imported Pixel has the necessary hardware, the carrier can still block these features, potentially preventing you from making voice calls entirely.