No app is required for calling. (That's way out of date-- Unreal has been true cellular voice for a long time.)
Hotspot: I can't confirm this is still supported, but the article you linked to does indicate it's supported. The article mentions "Premium" SIMs, which includes all current Unreal SIMs. (The Premium label was used to differentiate the old VoIP plans from the cellular voice plans.)
Historically, when it works, Unreal hasn't had a lower limit for hotspot use.
As I remember, early on hotspot use really was just not mentioned-- neither specifically okayed, nor specifically prohibited. I think AT&T branded devices may have had problems activating hotspot, because they wanted to 'phone home' for authorization. Factory unlocked phones, or branded phones from other carriers that had been unlocked didn't 'phone home' to check, and had no problem.