He didn't invent modern TV. He started with a 22 line mechanical system, based on Victorian tech. The last version (using CRT display and film based cameras) was 220 lines.
RCA & EMI developed modern TV based on a 1905 idea. USA was 440 (later 525) and UK was 405. Operational from 1935.
On a 9" screen (typical) it was indeed HD compared to 1920 x 1080 on a 42" screen today.
WWII saw higher resolution used for Nazi rocket range CCTV.
1948 saw 625 line in Europe.
Baird was a showman and not even a real engineer. Eventually the Baird company paid him to sit at home. The film based near real time camera with built in processing and scanning was clever and used in early satellites as electronic cameras were too poor. The film processed on the satellite could be high resolution and the scanning of it could be sent back slower than real time per frame, as it was really still photography.