Again, risk mitigation can’t be based on actual outcome (largely randomised), but rather possible outcome (defined probability).
Yes, it seems nothing negative came of him getting on the pitch, but we have concrete evidence that is not always the case, with invaders causing real issues in England over the last few years and definitely elsewhere in the world (again, just look at Brazil and Argentina, where things have been allowed to get out of hand regularly). And punishment in these cases is meant to be deference, because literally no club, in any game, at any time, can ensure sufficient prevention for individual pitch invaders, much less groups of them.
This is a case of systems analysis and management. You have to look beyond the single event and consider the further implications, even when it involves a fellow blue.
And, even beside all that, he knew a ban was the likely consequence of jumping on to the pitch and did it anyway. If he is a “real man”, he’ll accept the consequences of his actions.
There is nothing more Dipperish than knowing the consequences of your actions beforehand, deciding to take the action anyway, and then bemoaning having the consequences enforced on you.