Thank you very much for all the replies! I did the googling of course and it brought following result
https://taxsummaries.pwc.com/mexico/...ource%20income. stating that you indeed can be liable to tax regarding Mexican-sourced income even though not being tax resident. Of I understand that correctly of course, which I might not. So, it doesn't seem to be that easy as a 183day rule and I remember good old Boris Becker losing his trial against German tax authorities back in the days even though it was never in question that he mainly lived in Monaco and probably even less than 90 days per year in Germany.
That makes it dicey. Do online Poker winnings made from Mexico count as mexican-sourced income f.e.?