It is likely that in labeling the taxi horns as A, B, C, and D with circles, he was referring to the four horns, and not the notes that they played.
ɪɾ ɪz lˈaɪkli ðæt ɪn lˈeɪbəlɪŋ ðə tˈæksi hˈɔːɹnz æz ˈeɪ, bˈiː, sˈiː, ænd dˈiː wɪð sˈɜːkəlz, hiː wʌz ɹᵻfˈɜːɹɪŋ tə ðə fˈoːɹ hˈɔːɹnz, ænd nˌɑːt ðə nˈoʊts ðæt ðeɪ plˈeɪd.