Seeming mature to an adult isn't the thing in question though, is it? Not feeling or appearing awkward when interacting on their own in their 20s is what is being criticized. The anecdotal evidence you present doesn't include home schooled children in their 20s as far as I can tell.
Homeschooled kids have much more flexible schedules which can allow them to do things in the community during the daytime that are not available to kids who have to go to school in-person full time.
This can include volunteer work or part time jobs working with the public and interacting with people of all ages.
Why do you think you being forced into a monoculture of only kids your own age would help your interaction with others when you're in your 20s? 25 year olds don't behave anything like teenagers.
Because I've met several homeschooled adults over the years, and talking to them that's something most of them had in common when explaining the impact it had on their life. Looking for more objective data I found this one source that seems to be written by people not already convinced of the desirability of homescooling [1], forgive me for being skeptical of the objectivity of places called "national home education research institute". Overall it paints a more positive picture than I had expected, but also highlights it's limitations.
From those slides (I did have to open them via google, the direct link didn't seem to work):
Discussion
• Evidence from this study and others suggest that homeschoolers may not
be a socially isolated group
• Instead, homeschoolers in these samples seem to have peer networks
and social connections that arise in conventional and unconventional
social settings and they report being well-connected later in life
• It is important to note that although this study contributes to the
literature, it has methodological limitations (e.g. small sample, self-reports)
