Well, physics can not lie… will mean, with a certain amount of volume battery, you will get a certain amount of mAh.
Less brings you less, more brings you more… no place for witchcraft…
In my opinion, teclast lies where they can (crapy company) and so I think, you maybe have 6-7K mAh, but never 8K.
Very esay test, my 7K mAh spare battery from ALIExpress was taged as a 7K mAh and has around 6,5K mAh and last around 13h without new charging.
How is your battery doing (less than 13h or more)?