The answer to this is simply that all passive transponders, regardless of frequency, are RFID transponders… but NFC is a narrow set of standards laid on top of RFID. That means that all NFC “chips” or transponders are really RFID transponders that simply comply to the NFC standard, but obviously not all RFID transponders comply NFC standards. The Mifare chips are 13.56MHz like all (so far) defined NFC chip “types”, but it does not comply with the NFC standard so it is not considered an “NFC chip”, if that makes sense.
However, because people love to complicate things, NXP the company that makes the Mifare chips, also makes NFC reader chips. Many of those chips are included in phones, mostly Android phones. Because NXP makes the reader chips, and the Mifare chips, they created a pseudo-standard so that Android phones with NFC reader chips from NXP can also treat Mifare chips as “NFC”, meaning you can read the contents using apps like TagInfo and also write NFC data to them using apps like TagWriter… but still the Mifare chip is not NFC compliant so they are still unreadable by iPhones and Android phones with NFC reader chips from other companies like Broadcom etc.