The only way Sony could lock an Epic account is if the player, Epic, or both gave them the authority to do it as a condition of linking the account on PS4. Assuming that they did (because it is just an assumption at this point), here’s why they would want to do that, and it has nothing to do with being cocky.
Sony, MS, and Nintendo all make money off of third party licensing agreements that require game publishers to pay them royalties on games and content sold on their platform, sometimes selling consoles at a loss just to get gamers buying in their ecosystem, profitting on the back end on the sale of licensed content.
Fortnite being free to play means the profits for that game come from selling add on content.
This is where PS4’s dominance puts them in a different position from MS and Nintendo and makes facilitating the ability to use one Fortnite account across competing console platforms an unsound business practice.
Sharing Epic accounts across consoles is really about sharing customers. A PS gamer could be introduced to the game on PS4 and migrate over to Xbox or Switch, with those console manufacturers profiting from any add on sales made on their platforms.
The same is true vice-versa, but because PS4 would be offering a much larger customer base than MS and Ninty in such an arrangement, they would be more likely to lose a sale to one of their direct console competitors than to gain one.
This is aggravated by the fact that Switch has a fledgling online community and Xbox locks free to play games behind the Xbox Live Gold subscription paywall, meaning they would be contributing less of their own customers while Sony would be offering every PSN user (paid or unpaid) playing Fortnite. In fact, MS would directly benefit from their anti-consumer position of locking Live behind a paywall by not having to share all of their Live customers with their competitors.
I don’t see how Sony could justify to their shareholders engaging in such an unsound and one sided practice, which benefits their competitors by propping up their smaller online communities and feeding them customers.