That's the theory of the paper:

The weak influence of heritable factors suggests that there may be a major role for learning in both shaping and engendering synesthesia. Simner and colleagues tested grapheme-color consistency in synesthetic children between 6 and 7 years of age, and again in the same children a year later. This interim year appeared critical in transforming chaotic pairings into consistent fixed associations. The same cohort were retested 3 years later, and found to have even more consistent pairings. Therefore, GCS appears to emerge in early school years, where first major pressures to use graphemes are encountered, and then becomes cemented in later years. In fact, for certain abstract inducers, such as graphemes, it is implausible that humans are born with synesthetic associations to these stimuli. Hence, learning must be involved in the development of at least some forms of synesthesia.