And that is horrible. But there are a lot of folk who feel lonely from WFH who are not like that. The comment was insulting because it implied that if you're lonely from working from home it must be because you lack a social life. You can have a thriving social life and still miss the social aspect of an office.
I disagree, based on some of the other comments here and elsewhere.
There are people for whom being stuck in their WFH area for 8 hours a day 5 days a week with no social interactions is very unpleasant, even if after work they have a robust social / family life. Some of them were reporting this doing WFH prior to Covid.
The most amazing social life outside work won't compensate for that daily gap which occupies about half of their waking hours.
Some people need more regular personal interactions throughout the day than that.
On top of that, some people's work creativity or mental stimulation thrives on social interactions with colleagues in the same or related work domain. A social life with friends and family does not substitute for "work-oriented social interactions" with people who are grinding on the same kinds of problems, for example drinking buddies with whom you can discuss code. It doesn't really matter if these people are long term friends, as it's a different kind of stimulation.