By being in the business before this tech boom. Finance has been a major part of the San Francisco economy since the Gold Rush of 1849 (one of the 4 US Mints is here, one of the 12 Federal Reserve banks is here, and a bunch of other banks have important offices), and techies have been getting rich and moving here since at least when HP was founded, in 1939.
Why, and does it matter? I have no idea how rent control works. Clearly it's a silly, broken idea, because, I, too, know people of people who have moved into rent-controlled units and benefited from it. Whether it's legal or not, well, who cares. It happens, and it points out why rent control is stupid.