It seems to produce the odd effect that Tiger Warrior Training can't give two dots of a skill in a row to heroes, but can to extras. (Assuming the hero already spent all the XP before undergoing TWT.)
Why is that odd?
Extras are a game abstraction. They're not "real people" - they're a tool for the GM to handle unimportant characters more easily. There is, literally, by definition no such thing as an important extra, because if they're important they stop being an extra (even if they're not heroic - there are mortals who are not heroic mortals. A Solar's old mother who she sends shiny loot back to isn't heroic, but she's not an extra). In-universe, there's no such thing as an extra with a reduced number of health levels due to OOC narrative unimportance. It just makes stuff easier for the GM.
Hence, extra Tiger Warriors are sufficiently unimportant that it just matters that the Solar had a training arc lasting a season and now he's got a group of Tiger Warriors who are as good or better than the Realm's foot infantry. If they were more important individually, they wouldn't be extras. And as soon as a PC decides to start metagaming with that, the GM is entitled to hit them in the face with the corebook.