EDIT: I looked at the wrong instrument. See comments below to read about the MSS, "the oldest operational multispectral digital sensor"

This is what I see from a quick search. According to Wikipedia page Landsat 1 - Sensors:

ERTS-A had two sensors to achieve its primary objectives: the return beam vidicon (RBV) and the multispectral scanner (MSS).

And from Return-Beam Vidicon (RBV) Overview:

The Return-Beam Vidicon (RBV) sensor utilised vidicon tube instruments containing an electron gun that read images from a photoconductive faceplate similar to television cameras. The data stream received from the satellite was analog-to-digital preprocessed to correct for radiometric and geometric errors.

So... if I understand it correctly, it was an analog camera which signal was at the end converted to digital. But please correct me if I'm wrong!

ninja-EDIT: What happened on 1969, according to The First Digital Camera Was the Size of a Toaster - IEEE Spectrum:

CCDs [...] were invented in 1969 by Willard Boyle and George E. Smith at Bell Labs,

I read an interesting analysis: We as consumers just see pictures, but to them they were a chemical processing company. That didn't translate at all to digital pictures in any way like corporate experience, mindset, technical expertise, etc. It would have been hard to mentally make the change before it hit them in the face.

Then later they almost shut down their entire consumer film production because of digital cameras. So they were right in thinking it would massively impact them, but made some very wrong choices because of it.