If you read the decision then it's clear that it's very narrowly defined. It fits the circumstances and the laws in place in 2012. The Colorado anti-discrimination laws were tightened up since then and the Obergfell decision forced Colorado to legalize gay marriage. If Phillips were to sue under the current conditions there is no guarantee he would win.
The SCOTUS ruled that the owner was not in violation of discrimination based on refusing to make a cake to celebrate a homosexual marriage.
No matter how they or you parse it, this ends the ability to claim someone is discriminating against you based on their refusal to provide a cake to help you celebrate your cause.
Therefore it is not a narrowly based decision.