That effectively doesn't matter, they could have also gone for Linux or written their own microkernel and it probably would be transparent to the user. They did

That effectively doesn't matter, they could have also gone for Linux or written their own microkernel and it probably would be transparent to the user. They did choose FreeBSD because it's very safe, which the PS4's security track record has shown to be effective. Xbox now runs Windows NT, there hasn't been a great interest in hacking them due to low adoption and them allowing homebrew by default.