I think in this case, the Tizen OS (by Samsung) on these TVs has a developer mode where you can use their Samsung Debug Bridge CLI tool on another computer to connect to the TV and get a regular Linux user shell. From there, they found a local privilege escalation bug to get root. I don’t know the exact details but a lot of smart devices get poor security support, so you can just shotgun different known vulnerabilities and get lucky with one pretty often rather than needing to develop a novel one.