No, Core NFC is not limited to ISO/IEC 15693 tags at all. According to the documentation (Core NFC),
Using Core NFC, you can read Near Field Communication (NFC) tags of
types 1 through 5 that contain data in the NFC Data Exchange Format
(NDEF).
So Core NFC seems to support only tags that are NDEF formatted. NDEF is the NFC Data Exchange Format, an abstraction layer specified by the NFC Forum to abstract interaction with and data storage on different types of NFC tag hardware. There are currently five different NFC tag platforms specified by the NFC Forum and supported by iOS. Each of these tag types originates from certain RFID (NFC) tag products that already existed before NFC was born. They differ in their commad sets, memory layout and even the underlying RF protocol standard:
- NFC Forum Tag Type 1: This tag platform originates from Innovision (now Broadcom?) Jewel tags. It is based on ISO/IEC 14443-3A framing but does not use the anti-collision scheme from that standard.
- NFC Forum Tag Type 2: This tag platform originates from NXP MIFARE Ultralight tags. It is based on ISO/IEC 14443-3A.
- NFC Forum Tag Type 3: This tag platform originates from Sony FeliCa Lite tags. It is based on JIS X 6319-4 (and ISO/IEC 18092).
- NFC Forum Tag Type 4: This tag platform originates from NXP MIFARE DESFire tags. It is based on ISO/IEC 14443-4 (ISO-DEP) transmission protocol (both Type A and Type B supported) and ISO/IEC 7816-4 commands (APDUs) and file system structures.
- NFC Forum Tag Type 5: This tag platform was an agreement between features from ISO/IEC 15693 tags of different manufacturers.
Consequently, Core NFC supports subsets of all these NFC / RFID / contactless smartcard standards in the 13.56 MHz HF band. Nevertheless, you are bound to the features that are exposed though the NDEF abstraction layer. This means that you can only use those RF standards to read NDEF messages on supported NFC tags.
Also, while APDUs are used to access Type 4 tags, you are bound to those APDUs and those communication sequences that are used to read data from these tags. You can't freely control what APDUs are being sent.