Charitable undercover celebrities are the icing on the cake for Hollywood stardom. They're already famous and often also rich so when they give back to the community and do it in a funny way, it is really enjoyable.
Arnold is no stranger to the gym; the former Mr. Universe made his career initially out of being one of the strongest men on the planet. Since then, he has gone on to do a myriad of things including becoming the governor of California. Since that is no longer happening, the big Austrian needs to occupy his time somehow.
Arnold decided to invade a gym dressed as a new manager complete with mustache, hair piece and uniform to raise awareness for health and fitness. The accent is still there but even with the iconic voice, some people still mistook him, I guess being out of the public eye for a bit does that especially for a well 'stached undercover celebrity.