My screenshot is taken from the exact moment the television producers chose to pause.
Here is a higher resolution close up with a line drawn along the mow mark (if we assume that these are straight). The boots are clearly in line here and the fact is that the defender's boot is raised off the ground, grounded his foot would appear slightly closer to the goal line.
We are talking about mere millimeters here and without access to higher resolution images taken from cameras with a much higher frame rate there's no way to be more certain.
If even with blown up still frames we cannot be certain then you can be sure the linesman couldn't be certain in real time, in which case the benefit of the doubt goes to the attacker.