Yes, and I also think that the way people are asked makes a difference. There's still a huge stigma about people (particularly women) admitting they might regret having their children. The number of people on this thread has surprised me, and I think the anonymity of an online forum might have something to do with it.
Until very recently there wouldn't have been any method of doing an (almost) completely anonymous study. Even 'just' admitting to a real person, albeit a complete stranger, that you might regret your kids, would be harder face to face or even over the phone than typing it online, plus if they had your contact details there's always the miniscule worry said kids could find out.
Not to mention that even 'only' twenty or thirty years ago things were still very different in terms of women's careers, maternity discrimination etc. The number of people without kids was much lower so it was harder to envision what a happy childfree life might look like. Because it was rarer, people were more judgemental, etc.
Also, as pps have pointed out, you only know your exact circumstances. People who have had kids will have also had a period (even a short one) in their lives before they had kids so can make a vaguely educated guess about what their lives could have been if they hadn't. But you only ever experience having your own kids. Someone who says they don't regret having them might say differently if they'd had different children and vice versa.
Also might change depending on when they were asked - someone might say 'No! Never! Best thing that's ever happened to me' at baby stage, then 'God yeah, if only someone had told me it was this hard,' when in the trenches with teens, then be back to not regretting it when the DC are older and have given them grandkids.