What bothers me most is even if they wanted to interpret the rule that way, why does it matter if a shoulder or head is offside if neither the shoulder or head are used in the build-up/goal? If Isak had sent a shoulder pass across to Burn, then sure whatever it's offside, but his shoulder played no part and gave no advantage.
That's just giving them the benefit of the doubt when it really should just be feet and that's it.