The result has been consistent with McLaren's pace this year, a stark contrast to its 2018 season when it was competing towards the back of the field.
"No one expected in the team, finishing last season as the ninth fastest car, to make such a step," Seidl confirmed.
"It simply shows that a lot of the changes triggered last year are paying off. The team did a good job bringing this car to the track. Also the drivers from last year, we should pay a lot of credit to, Fernando and Stoffel.
"They pointed out the weaknesses and where to develop the car. And now the target is to keep going with the development."
Sainz pulled his McLaren into the top ten after starting from the back of the grid due to engine penalties.
Seidl believes that the Spaniard could have overtaken Pierre Gasly for seventh, had he not picked up front wing damage on the final laps.
"It was great to finish P6 and P8," Seidl said. "It was a great drive from Carlos to recover after starting from last. It could have been even better but unfortunately, he damaged his front wings some laps before the end.
"Then it was about surviving and keep the position. There was a chance to attack Gasly.
"We had a lot of understeer. In terms of lap time, we lost about a second. We were lucky that Raikkonen also lost lap time.
"I'm very happy for the team, because it's good to confirm that Paul Ricard was not just a one-off. We were once again the fourth quickest team in the midfield."
Bahrain International Circuit - Winter testing