In fact the author is describing the opposite situation, where his "LTE calls" require the international prefix while the "Wi-Fi" calls do not require it (not sure if VoWifi or plain VoIP, it's not clear from the description, and I don't know of any phone provider which allows true VoWifi while roaming).
Note most modern dialing programs (and likely all of the VoLTE-capable ones) will silently add some prefix to your calls anyway, so it's really hard to know what's going on behind the scenes. But you usually can't dial your provider's special phone numbers "as if your calls where backhauled to their network through VoIP" when you are roaming, even with VoLTE. To e.g. check voicemail you will still need to dial your provider's roaming-specific number, which will come with an international prefix.