No, only *badly* nested tables
I'm quite confused about this error. Is it for every nested table? Aren't nested tables sometimes legitimate?
Also, the current doc says "you might prefer the new rendering". If it's a possibility, then why is this lint errors listed under "High priority"?
Or am i misunderstanding something?
It is for every *badly* nested table. If a table is properly nested, it is inside a <td> cell. If it is improperly nested, it is usually inside a <tr> tag which is when this linter detection triggers. Tidy and RemexHTML/Parsoid handle this improper nesting differently. Tidy removes the badly nested inner table. RemexHTML closes the outer table. So, yes, the fix depends on what the editor intention was.
Please edit the help page based on this explanation to clarify any confusion. I am sometimes a bit too close to the problem / solution to do that adequately.
Can it also happen with tables that are not nested?
On the page w:he:מחוזות אינדונזיה I don't see nested tables at all, but it appears on Special:LintErrors.
If you click the edit link on the linter page, it says https://he.wikipedia.org/wiki/%D7%AA%D7%91%D7%A0%D7%99%D7%AA:%D7%9E%D7%A4%D7%AA_%D7%9E%D7%97%D7%95%D7%96%D7%95%D7%AA_%D7%90%D7%99%D7%A0%D7%93%D7%95%D7%A0%D7%96%D7%99%D7%94 is the source of the issue.