In plain english, the datasheet says that THE ESP8266 IS 5V TOLERANT! And probably the manufacurer implemented a snap back rather than the ESD diodes to ease the interface with 5Volt systems ...

Some people state that ESP8266 is tolerant of 5 V logic levels on its GPIOs, while others vehemently disagree, pointing at the datasheet-stated 3.6 V maximum. Datasheets aren’t source code fo… ...