> I never used any LLMs, even for searching

I highly doubt there is any mainstream search engine that doesn't use any LLM like technology in the backend to provide search results.

> I checked it with a couple of AI detection tools and they confidently said that 60% of the article was written by AI.

These tools are as reliable as the polygraph. I have seen passages from books published before 1900 getting a 90% likelihood of being AI generated from these tools. At the end of the day there are only so many ways to write a grammatically correct sentence and if an AI can write it, chances are some human somewhere already did.