I think part of the problem is that our perceptions of "beginner", "advanced" etc change as we learn any task, so a lot will depend on our own current vantage points. A beginner who just bought their first piano and is trying to get their hands to obey at all and to figure out how to read music may understand beginner to mean someone who is in their first few months of learning and feel very intimidated by posting in the same threads or recitals as people who have been playing for a couple of years thinking that anyone who is past their first method book is no longer a beginner.
Others may think in terms of lessons for school age pupils, typically given over about a decade, where that decade is roughly divided into three thirds referred to as beginner, intermediate, and advanced levels.
Others yet may have played leisurely for decades, but peeked into the pianist corner on that other site and concluded that it seems to be the place for people who have studied at conservatories and in many cases are set at perfecting their playing in order to become professionals or who already earn their living playing piano. That's definitely not the place for them, they say, as they see themselves as eternal beginners or mere hobbyists and are totally happy playing a relatively easy repertoire. Playing an instrument for say a decade may seem like a very long time if it's all in front of you, but not so much so in retrospect.
In other words, a very big range of people considering themselves beginners. And they are all correct by their own definitions.
By defining beginner very narrowly, there may be a large number of people who feel that they will forever belong to the void between beginner and pianist and that there is no place for them. If defined too narrowly there may also not be enough members to keep the beginner forum lively as a constant flux of new members will be required.
Then there is the additional complication that adults who are new to the piano may have half a century of experience on another instrument, in other words not beginners at all when it comes to music or instrument learning in general.