I've started doing everything the article said but people constantly ask if I'm "okay." Do I respond with yes, or what? I want to keep the stoic thing a secret.

No reason to keep stoicism a secret. Be honest and forthright. Stoicism isn't meant to make you appear grumpy or sad, or emotionally dead inside. It's meant to give you inner strength, self reliance, and independence from the need to be emotionally validated by others, because you're now able to emotionally validate yourself. It's possible people are noticing a change in your behavior, and are just checking in. If that's the case, be appreciative of their concern, let them know that you're alright, and further let them know that you have inner strength to spare if they happen to need support.