This is too complex of a topic to treat in full with all its due nuance without it being the primary focus, but I will give a basic starter of the subject for clarity's sake here based on my own current understanding. Calling a deity an aspect of Tezcatlipoca means that to some extent they're not considered a fully distinct individual from Tezcatlipoca. Think of Peter Parker and Spider-Man, those names bring up different images to an extent but also you're thinking of the same person, just different expressions of their identity.
Syncretization on the other hand is the merging of distinct figures together, and not always as unsubtly as sticking a hyphen between their names. Sometimes it's just the transferal of duties or relationships. For example Tezcatlipoca has been subtly syncretized with several Christian figures through iconography and the timing of festivals.
But what's most important to remember is that mythology is fluid. Mythological categories are not binary, except for things like distinguishing between genuine parts of a religion's evolution and the inventions of pop culture based on its mythology. Aspects of a deity can come to be seen as separate beings or a deity can come to be seen as a manifestation of another.