That's clearly not true. He wasn't eligible to be checked, which is why it was an interference penalty.
Let's be clear, "just outside" is coming from you, not the ref. Ref assessed it a major btw, just like Hagels. And Hagel's hit also has ifs and buts if that's what you are looking for. If Barkov played the puck as he intended to, it would not have been a penalty.
People keep saying this, but nobody yet presented any explanation as to why this "major" difference is relevant to suspension. There is certainly nothing in the rule book.
Unlike that never eligible/not eligible hair splitting business, this explanation has some merit. In the sense that head contact was a factor in previous suspensions. However, so far it has been only if the head is "primary" point or a "target". If this relatively minor head contact were truly the reason for Hagel's suspension, we should expect the NHL to start suspending for interference when there is only incidental head contact. That would be a major shift, so unlikely to just introduce this change without prior announcement and rule amendments.