Characters in Jurassic Park are portrayed as flawed, imperfect people who make mistakes. None of the plot relies on them being idiots or anything, but people screw up, panic on occasion, and don't know things from time to time.

Dr. Grant using a stick to test the fence is a mistake, albeit a small one without real consequences. While it doesn't distract from his character arc of how he feels about kids, it is his character simply messing up.

I also disagree with the person you replied to. While their assessment is correct, Dr. Grant is a character with a lot of time working in the field and therefore has a lot of practical skills. He does way, way better than a doctorate in mathematics working in academia would. Writing off all people with a doctorate (or experts in general) as being hyper specialized is a mistake.