So have I. They are two separate companies. The current Stern Pinball does own the IP of Stern Electronics, but they are separate companies. I believe this is where your confusion is coming from.
Stern Electronics was created out of the ashes of Chicago Coin in the late 70's, which had gone bankrupt. Gary and his father Sam, bought up the assets of Chicago Coin and created Stern Electronics, which folded in 1984.
Gary Stern created a company called Pinstar that sold Bally conversions kits. Then, in 1986, with funding from Data East, he opened a new Pinball company. This was sold to Sega for a few years, and then Gary bough the company outright in 1999 and renamed it Stern Pinball. It is this company, Data East/Sega/Stern Pinball, that is celebrating it's 30th anniversary.