When researchers say synesthesia without qualification they mean the developmental kind: present from early childhood, stable for life, and clustering in families in a way that points to genetic predisposition without any single gene found.
Childhood learning shapes the content while the trait supplies the mechanism. Colored letters need an alphabet, so the pairings form as literacy does, then freeze. Which explains the odd specificity of adult synesthetes’ inventories: the trait is inherited, the vocabulary is learned.
The qualifier exists to separate this from acquired and drug-induced forms, which differ in origin, stability, and character.