This subject has been brought up before.
Apparently, it was quite common for the comics companies
at the time to assume that when one person bought a comic
they would pass it on to several of their friends to read, so it was quite common for them to multiply sales by 3, 4 or even 5
in these blurbs. So 5 million readers may equate to 1 million copies sold. Still no mean feat, though.

A quick scan through Atlastales for other titles of the same vintage shows no other title making a similar boast, so there
must have been something to it.