No, it’s not robust. It may work for your TV, but it can be worked around.

DNS is like a phone directory for Internet: it translates domain name to IP addresses. If you block the DNS (what pihole does), it blocks the directory access. But if the IP address of the servers are hard-coded in the firmware, the TV does not need a DNS, it can reach the server directly.

To trick the TV, you need to restrict the IPs it can reach. It might be delicate: it probably tries to ping some comme IPs to check it’s connected, then call the brand’s server for ads/updates/etc.