IANAL, but the way copyright works in the USA is based on what's known as the inverse ratio rule. Generally, two things are required to be established in order to be liable for copyright infringement: access and substantial similarity. But the more access you have, the less substantially similar you need to be to be liable.
Replit may well have been within their rights to sue this person for creating a product like theirs after he was an intern at their company... irrespective of how many other similar products were out there. His greater access may have lowered the threshold to the point where any similar competing product of his may have incurred liability.
IANAL either but I doubt they would have sued over copyright when there's probably much "better" options. Their "top lawyer" would have probably found some kind of predatory IP transfer ("every idea you've had during your time with us is ours in perpetuity" kind of thing) or non-compete clause in the intern contract. These kind of clauses I hear are SOP and I doubt this guy had the foresight fresh out of school to scrutinize the contract and negotiate anything better.
You worked for a company and then made something really similar? idk, if what the author did was right or wrong legally, but sounds like a dick move. Just build literally anything else.
I could see this if he made something dramatically more complex than just running python online. The author shows pretty clearly that nothing he is working on isn't already out there.
Replit may well have been within their rights to sue this person for creating a product like theirs after he was an intern at their company... irrespective of how many other similar products were out there. His greater access may have lowered the threshold to the point where any similar competing product of his may have incurred liability.