No, it is not a meaningless distinction. You are using underpaid as a take on supply/demand problem, whereas I am using underpaid as a person's personal take on

No, it is not a meaningless distinction. You are using underpaid as a take on supply/demand problem, whereas I am using underpaid as a person's personal take on their salary. If the supply/demand curve showed that teachers should be getting paid around $20k/yr, they would almost universally claim to be underpaid, despite being able to fill the open positions. Saying "Well, you're not actually underpaid" means nothing if people feel like they are underpaid.