The difference is that you can customize/debug it or not. You might say that a .EXE can be modified too. But I don't think that's the conventional definition of open source.

I understand that these days, businesses and hobbyists just want to use free LLMs without paying subscriptions for economic motives, that is, either saving money or making money. They don't really care whether the source is truly available or not. They are just end users of a product, not open-source developers by any means.