I read your post twice and it didn't come across as smug to me. I think you just touched a nerve with that poster.
I get it, OP, and yes, I'd likely be in the sane boat.
I have 'friends' but most of them are really close acquaintances. Of my actual friends - one is retired and in a new relationship and has disappeared off the face of the planet 😅; one has caring responsibilities for an elderly parent and another for grandchildren; others just have very different work patterns which make it difficult to meet up as we have to work around work and not just when we want to see each other. Many of these people have their weekends booked/planned weeks/months in advance or have regular commitments.
The few people who would be more available aren't people I'm as close to and would probably be surprised that they were the person I'd chosen to ask to see if I had some rare time to myself. And will have already made plans with their actual friends (past experience of asking tells me this). I wouldn't be the first person they'd ask either.
Like you, I'd have no problem filling the time doing things I enjoy - a walk in the local park, bit of self care, some exercise, reading, take myself out for lunch, lie in bed, find a pub and go to see a band on my own.
But it wouldn't be a choice. It would be my only option and its not quite the same. Even if I enjoyed every second of it.
I know others are saying they'd love the opportunity and it would be a luxury and I'd agree, it would feel like that if you were choosing it but when you've got no alternative, or don't feel that you have, it lands differently.