Early universe's first starlight (indirectly) detected for the first time, could it have been detectable with 1970's technology?
I could not find a freely accessible version of this article, so I am mostly guessing here.
From this article https://arxiv.org/pdf/1206.0267.pdf (especially sections 4 and 5), the main challenge seems to be distinguishing between the different components of the signal:


  (1) the 21 cm signal coming from the high redshift Universe,
  (2) galactic and extra-galactic foregrounds,
  (3) ionospheric influences,
  (4) telescope response effects,
  (5) Radio frequency interference and
  (6) thermal noise
  Distinguishing between all these components means understanding each of them equally well and probably requires recent knowledge (in cosmology, radio receptors, etc)


In any case, the idea of using the 21-cm line to probe the early universe seems quite old:
https://academic.oup.com/mnras/article/188/4/791/999622