It is impossible to be made less smart by not manually writing code. If it were possible, you would be stupider every time you go on vacation. Your brain's neuronal connections do not die from talking to a chat bot.

What's actually happened is you took a break from a highly technical skill. Every person on the planet will "forget" some part of that technical skill if they don't use it in a while. But the information is not gone, it's just been de-prioritized for other more pertinent information. The information comes back once you give yourself a refresher.

Before AI, it would be months between me writing a full program in one of several languages. I would forget simple things like how to start a function definition. But I did not really forget, because after a quick glance at an existing function, I remembered all the other possible syntax in the function definition. There's no need to panic, your brain is working normally.