The USB AMP meter used on an iPhone is really only good for knowing if the phone is charging or not. You can get an idea of the battery charge by the amount of amps it is drawing. I think there would be too many variables to determine the true battery health.
When using a battery tester the battery connector goes straight on the PC board and reads information directly. When using a USB AMP meter it goes through a charging cable, into the charging port than through the charging circuit on the logic board.
On a good battery and logic board you may be able to tell the health but if you determined the battery heath was way down using your AMP meter it could also be a faulty cable, charging dock, gas gauge circuit or one of the charging IC’s.