Explanation
The overlap is larger than the discourse suggests. What genuinely differs between the three is the text fields rather than the video: YouTube leans on the title for search far more heavily than the other two lean on captions, and each platform places its interface furniture differently, which affects where subtitles can safely sit.
There is no penalty for posting the same video in multiple places. The penalty that does exist is for another platform's watermark, which is avoidable by exporting the clean file rather than re-saving from the app you published to first.
The common mistake
Cross-posting between your own accounts on the same platform. Overlapping audiences notice immediately, and it makes both accounts redundant rather than doubling their reach.