I don't think they know for sure how many escaped. This is a quote from another article .
"Cooke, in an estimate to WDFW, put the number of escaped fish at 4,000 to 5,000, according to Ron Warren, fish program assistant director for the WDFW." 
I think the three hundred thousand number might be the total they thought they had in the net pen.
I can't imagine people wanting to eT fish that have been treated with antibiotics and who are fed growth hormones. They are truly Frankenfish !