Just want to post this again and explain how the clarified rules (which are posted on WotC's website) answer most of the questions in this thread, in case there is any confusion. Here is the text again:
A long rest is a period of relaxation that is at least 8 hours long. It can contain sleep, reading, talking, eating, and other restful activity. Standing watch is even possible during it, but for no more than 2 hours; maintaining heightened vigilance any longer than that isn’t restful. In short, a long rest and sleep aren’t the same thing; you can sleep when you’re not taking a long rest, and you can take a long rest and not sleep.
Here's what we learn from this:
- The reference to 2 hours in the PHB only refers to standing watch, not to the other light activities.
- You are not required to sleep at all during a long rest.
- You can sleep and not be taking a long rest, as sleeping and a long rest are not the same thing.
- It has to be at least 8 hours long, but can be longer.
Going back to the message the OP posted: Can a PC choose to NOT Long Rest even though they are not engaging in strenuous activity for 8 hours?
I think the rule answers that WotC posted clarifies that a PC can choose to NOT take a Long Rest, as the final sentence posted above make it seem as though the player willfully chooses when they are taking a Long Rest, and not all periods of downtime are automatically Long Rests. That's my takeaway, but opinions could vary. As far as RAW is concerned, I think there isn't any doubt that a player can be at ease for 8 hours or more and not be taking a Long Rest.
Naturally, a DM can choose to not take any of this into account and continue to do as they wish at their own table.