"41.1 Boarding – A boarding penalty shall be imposed on any player who checks or
pushes a defenseless opponent in such a manner that causes the opponent to hit or impact the boards violently or dangerously. The severity of the penalty, based upon the impact with the boards, shall be at the discretion of the Referee."
The rule is clear, if a player is pushed dangerously into the boards then it's boarding. Oleksiak clearly extends his arms in a pushing motion, thereby fulfilling the rule as written.
Whether one judges the play as dirty or not does not factor into the assessment of this penalty, or whether the player is wobbling on his skates- all this is irrelevant. Even Oleksiak stops after Lehkonen hits the end boards because he knew he pushed the man into the boards.
Thank you for re-posting the video which clearly shows Oleksiak pushing Lehkonen into the boards.