They did try to put a baphomet statue in front of a 10 commandments monument in Arkansas. They are fighting for a plurality of religion, not secularism.
If I wanted to contribute to a secular cause, I would much rather contribute to a secular organization to begin with.
TST does not proselytize
But the only times you hear of them is when people are trying to get more folk implied (or when they send a lawsuit, but that's an other story). TST plays the card of a non profit when they don't want to be associated with religious weirdos, and the card of religion when they want a special treatment. In the end it's a knockoff religion that hijacked the name "satanism" while replicating what they denounce of christians.
Fire is bad so we need to recruit more firefighters to fight it.
It's much closer to putting up advertisement against advertisement.
They are just an NGO.
That's not true. It's a bunch of for-profit organizations coupled with a recognized nonprofit church so they can be exempted from taxation. See here : https://the.satanic.wiki/index.php/The_Satanic_Wiki . Also, as a supposedly non-profit org, they do not disclose their financial information, which is usually a big red flag.