The ball is played from the byeline in a corner. For any player to be ahead of the ball they’d need to be on the goal line and the corner taken from the back of the quadrant.
Was the player called offside standing on the goal line?
No.
The corner taker played a short corner, and started running back.
He was called offside, when running back past his teammate to collect the pass back to him.
His heel was offside, as it was ahead of the ball, and the defender.
It is correct, it turns out. But seemed odd as I don’t think I have ever seen an offside call like it.