Hi Nicolas Boucquey,
The power supply for IO pin and reset pin are different. There are diodes connects with VBAT which protect VBAT. Thus we don't need worry voltage on reset go to VBAT. We only need consider reset pin pad itself -- 5V tolerant is ok.
While for IO pins, there is no such diodes exist. So we define 3.6V tolerant if VDDIO doesn't exist.

Have a great day,
Jun Zhang
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