@clearstream and
@Rune: are we at least in agreement that a character must accumulate 8 hours of downtime to complete a long rest, even if we radically disagree about what "downtime" means?
If so, can we also agree that you successfully complete a long rest if you accumulate eight hours of downtime (including six hours of sleep) before you accumulate one hour of Strenuous Activity?
My problem with this reading is that it renders the statement that you can engage in no more than 2 hours of light activity redundant. You could remove that text entirely without changing the meaning, which indicates to me that probably isn’t the intended meaning.
Personally I'm ok with a few extra words for clarity, both because it's not a technical document and because when writing errata it makes sense to err on the side of redundancy.
Additionally, it seems more likely to me that the designers used a few extra words for clarity than the idea that they wrote two lists intended to be merely examples (in the same sentence) but only included the necessary language "such as" in one of them.
Finally, your interpretation lets Strenuous Activity counts as downtime. Given that Strenuous Activity would not be included in downtime in natural language, your interpretation requires treating "downtime" as jargon. Although the designers were wildly inconsistent with the use of natural language vs jargon, I'm still going to favor an interpretation that lets terms keep their natural meaning.
You're reading things into it to get to that conclusion. Sitting and staring at a night sky is downtime, but not light activity. Light activity is doing something active, but only lightly active. Reading, talking, eating, standing watch, playing poker, etc. Looking up is inactive, but not asleep, yet is still downtime.
I'm reading "activity" as being used in contrast to "sleep". That appears to me to be the most natural way to read the usage in this context. I understand that you disagree.
5e is not prescriptive like that. It doesn't list everything that is light activity or exertion or non-activity that isn't sleep and is still downtime. It leaves that to the group. While there are things that are lightly active that aren't on the list, such as playing poker, shooting rubber bands at flies and typing on a keyboard about what light activity is, that doesn't mean that inactive things such as staring at the stars are not also downtime. They just didn't need to tell you that if you're inactive, but not asleep, that it's downtime. They were pretty sure you could figure that out on your own.
I agree that it doesn't list everything that qualifies as light activity. We know that because the designers used the phrase "such as" when providing the list of activities that qualify as light activity. I think it's significant that the designers did not use "such as" earlier in the sentence when they provided the list of what a character does during downtime. The designers clearly know how to denote a non-exclusive list, and they didn't do so when listing what a character does during downtime, instead listing only sleep and light activity.
So yes, because I agree that the list of what qualifies as light activity isn't exclusive, I naturally agree that there are things other than the provided examples that qualify as downtime.

I just think they're downtime because they are light activity, whereas you would classify them as something other than sleep/light activity/strenuous activity.