usdt tether market cap grows from 4,6 billions to 10,800 billions from March 2020
For last week - when btc jumped from 9 000 to 11 000 - tether issued 800 millions usdt
Do you think tether really has fiat for backing this usdt?

I do but it's not all of tether has already backed by the dollar. I remember in the site of tether and it's clearly stated that it's not all of USDT that has already issued by the company was backed by dollars. You can try to visit the site too right now.
The growth that happened with tether is really fast compared with another stable coin. hmmmm that's interesting.