Basically it explains that SLAAC RFC itself does not define the /64 limitation, but other RFCs that are relevant to network operation do.
"""
The addressing architecture [RFC4291] [RFC7136] sets the IID length
at 64 bits for all unicast addresses and therefore for all media
supporting SLAAC. An immediate effect of fixing the IID length at 64
bits is, of course, that it fixes the subnet prefix length also at 64
bits, regardless of the aggregate prefix assigned to the site
concerned, which in accordance with [RFC6177] should be /56 or
shorter.
"""
https://datatracker.ietf.org/doc/html/rfc7421 is helpful in understanding this.
Basically it explains that SLAAC RFC itself does not define the /64 limitation, but other RFCs that are relevant to network operation do.
""" The addressing architecture [RFC4291] [RFC7136] sets the IID length at 64 bits for all unicast addresses and therefore for all media supporting SLAAC. An immediate effect of fixing the IID length at 64 bits is, of course, that it fixes the subnet prefix length also at 64 bits, regardless of the aggregate prefix assigned to the site concerned, which in accordance with [RFC6177] should be /56 or shorter. """