Isn't 50TB just 6 years of videos, in even the best case (1GB/h)?

They weren't recording in 4K last century.

One hour of 1970's-grade NTSC video is around 300MB.

More to the point, it's not like they archived 70 continuous years of video. Many local PBS stations produce an hour or less of video each week. This being a St. Louis station, however, it's probably somewhat more.