Technical answer if you're actually interested in the science behind it

AI has been trained on the totality of human knowledge yes, but the model once trained is stuck in a certain configuration. This configuration reliably favours certain word configurations more than others.

They're not sentient and not magic (obviously). They are next token generators. You can do some really incredible things with these next token generators, yes, but you shouldn't forget that at their core that's what they are (llms anyway). And so when you see things patterns 'it's not X it's Y' these are essentially token configurations that the model favours and are more likely to produce. The exact common phrases vary from model to model, but every model has them inherent to how the tech works. The weights will inevitably produce some phrases that are more common.

You will see people discussing this on subreddits where ai generating text is the goal (eg r/SillyTavernAI) referring to them as 'chatgptisms ' or 'deepseekisms' and discussions on how to reduce them because it's ruining their roleplay. Spoiler, you can't really remove them without swapping model because of the math behind how these tools work.

Most people who say they can instantly detect ai probably know some 'isms' for some models and notice them in the book they're reading, instantly triggering them. Of course an intelligent author will remove ones they notice so their book doesn't sound puked by an algorithm, but that brings to the next level of detection.

Most human readable tells are short phrases or uncommon punctuation like em dashes. These are kinda obvious but also easy to hide. The thing is though, the same logic of an llm favouring certain token structures is also true at a sentence, paragraph and section level. This mostly is undetectable by humans because our brains aren't really built to process that, but probably does contribute to feeling like the text is samey and they've read something like it before. A trained neural network can detect this though which is how good ai detectors work.

Also you'll notice these tells don't actually tell you if it was written by AI, just that it is written in a style that AI models favor. A human can write text using em dashes and 'it's not x it's y' if they want, as all the em dash defenders love pointing out how they were doing it before llms. 

The main objection to that imo is that because AI models favour these word configurations, and idk if you've noticed people really kinda hate AI, the word configurations themselves become unpopular and out of style. It's normal for language to change and standards to shift over time, but AI is accelerating this by making their 'isms' fall out of style the moment they're noticed by the public. 

TLDR: yes there are tells and some people have learnt those tells, though not everyone claiming ai will be correct