As much as I understand the point you are trying to make, which actually can and would have been true and justifiable only if bitcoin and tether were a product of the same company, or under the same company.
But the fact that Bitcoin is a stand alone crypto currency that is not controlled by any company or individual, make bitcoin imunune to any form of price inflation, tether has and can't inflate the price the price of bitcoin, if they need bitcoin, they have to buy it from the open market just as everyone does, and for them to make any profit, other  investors have to also invest in bitcoin too, which means that when the demand for bitcoin becomes high, it's price rise according to the level of its demand, tether has no way of inflating the price of bitcoin, before you even imagine this, you would have first tried to find out how many bitcoin tether (as a company) has in their custody, and also check at what price they bought it, then compare that to the current price of bitcoin, then tell us if it was tether that inflated it's price.