That BI article is confused. The title says "here's how long the average employee stays at the biggest tech companies" (retention), but then it goes on to give

That BI article is confused. The title says "here's how long the average employee stays at the biggest tech companies" (retention), but then it goes on to give stats about average tenure. If your company doubles in size every year, then your average tenure is going to be very low--most people just started recently! You simply cannot use average tenure as a proxy for retention or turnover. The OP makes the same mistake, which is where I stopped reading.