I figured it out.
For whatever obnoxiously strange reason, Apple will not let you delete the standard Icloud Email account on an iPhone or iPad.
You can delete any other email accounts you may have but you may not remove the icloud one.
I backed up the phone and reset it back to factory settings, then I restored the backup. Now the mail folder reads 100mb while the 30,000 emails are still in the icloud mail server. Everytime you open an email it downloads it from the server.
10gb of "opened" emails storage space is now reclaimed off this iPhone by restoring it to factory settings.
Why you can delete any email account but not the icloud email account is anyone's guess. But as I'm sure the entire planet will agree it's a ridiculous design choice.
If you want to cleanse the iPhone of all opened emails to reclaim 10gb you have to delete the ENTIRE phone and restore it from the backup. The emails will still be there, just on the server instead of downloaded on your phone.
Anyone that tells you otherwise or pretends that you can delete the iCloud email account off the iPhone to reclaim storage space is lying to you.
I don't know why I bother with Apple and their bizarre functionality limitations on electronic devices I buy from them for close to $1000.