Satoshi did not technically solve the byzantine problem, merely solved it in a probabilistic or pragmatic manner with game theory where someone is incentivized to secure the network instead of attack it. Sybil attacks can still occur by a persistent and motivated attacker but they are extremely expensive and because of PoW(vs PoS/PoI/DPoS) there are signals that can pre-emptively leak of a potential or upcoming attack. (I.E... If the NSA wanted to attack bitcoin they would need to either build a secret ASIC factory or start purchasing ASICs in large quantities. This would in turn signal a concern to the manufacturers and the greater community with unexpected rising ASIC costs or a leaker would let the community know of the secret ASIC factory)

With PoS/PoI/DPoS a sybil attack can come without any notice and with potentially much cheaper costs. (No, an attacker need not have to "buy" coins to attack, They can create an exchange/bank that pays interest/dividends to corner a good chunk of coins 5-30% needed depending upon the algo, Or they can create a popular wallet with a backdoor, Or they can compromise several large bagholders computers, Or a few large holders could short and attack their own coin, ect..)