> It’s not the same as viewers watched, which was helpful to understand the popularity of a show relevant to other shows in the same time slot

Indeed it was. The world was easier to understand back when all networks agreed to air their shows on a comparable format. But nowadays with Netflix doing full binge releases and other studios doing weekly releases at a variety of times (HBO Max 9pm EST, Amazon midnight, Disney+ 3am) the old metric doesn't work anymore.

If you want to measure streaming audience in "number of viewers" then let me ask you this: Say Netflix released 8 episodes at once with 8 hours of run time. How much watch time should an account need in that week to be considered a viewer?