Absolutely not okay. ADHD medication should only be taken if one has ADHD. Which can only be determined by a medical specialist. And even if you do have ADHD, you don't have to take meds. ADHD meds are often misused as drugs, so suggesting this is basically asking you to take drugs. Not okay.
If a manager has a hunch that one of their employees has an unknown medical condition they might tell them about it. If their relationship is good enough.
If, on the other hand, they notice a performance issue, they can talk about that and potential ways to fix this. Which would include a quiter workplace, more interesting tasks or an inquiry of underlying issues.
I have ADHD and would have been happy if I had been diagnosed earlier. But I became a Machine Learning engineer without diagnosis. And while meds are nice, they surely were not the end of my problems. I am glad you don't work there anymore, because the real problem there was this manager.