This isn't a hack, it was straight arbitrage. I distinguish them because there was at no time a transfer of administrative power or control over the contract or targets infrastructure to Medjedovic.
In a smart contract, I'd make a legal distinction between syntactic parsing and calculation, which has to do with the purity of functions and data. An arbitrage would be fair game if it levered an unanticipated calculation, whereas a recent example where the contract was only checking the last several bytes of a destination address key would be a parsing exploit. Medjedovic's arbitrage as described appears to be a pure calculation advantage, and not exploiting a parsing error, and so this is very reasonably fair game.
He used logic endogenous to the contracts, with no exogenous control of the systems running the contracts. When you exploit a buffer overflow, you are breaking through (sabotaging) a parser as a means to manipulate the raw memory and machine - whereas this arbitrage is closer to something that lies somewhere between clicking on a link someone provided but had some unspoken intention about you not using it, and a SQL injection or other evaluation error that yields an index. (edit: Actually, it's more like saying something really funny and unexpected on a platform that hasn't banned that kind of humor yet, and they're just mad about the consequences. we could even see a future where the distinction between a hack and arbitrage will be the complexity class of the algorithm and whether it represented a scheme that was Turing complete)
Unfortunately, in Canada they'll go after him just as a fugitive now, and there is no shortage of political actors who will want to make him the perfect example villain for their hysterical policy objectives. This is one of those increasingly classic situations where a really smart kid gets system-involved and can't comprehend how insane it is because the legal system and politics are not subject to mere reason. If he has the money, fleeing before charges were laid was probably even rational, as there is no reason to expect the legal system is equipped to deliver justice in something so new.