I don't believe that "Almost half of professional developers contribute to open source projects.", even if you count every single half baked unfinished project on Github as contributing to open source.
Yes, but even there the 50% seems too much. I can see people submit that much bugs in cutting edge communities (where there are still a lot of new bugs to submit), but when you work with something stable you rarely ever need to do so. There is huge amount of positions where people successfully use what exists.
Plus, calling bug submissions as open source contribution is a stretch. You would not call "contributing to close source" when someone reports a bug to a company.
I have 3-4 public github repos ( <20 stars ) and I sometimes participate in definitely typed typescript moderation. I remember that I checked “participate in open source” on that question
I'd be willing to bet that people that answer surveys about this are generally more involved in other areas online like OSS.