While alethes (H227) means 'truthful' (describing someone who tells the truth), alethinos means 'genuine' — the real article as opposed to a copy or shadow. It is a word of ontological quality: not 'this statement is true' but 'this thing is the real one, the authentic fulfillment of what shadows and types were pointing toward.'
John's Gospel is saturated with alethinos. Jesus is the alethinos light (1:9), the alethinos bread from heaven (6:32), the alethinos vine (15:1). Each time, a contrast is implied: there were other lights, other breads, other vines in Israel's story — but Jesus is the authentic, ultimate reality they all prefigured. Hebrews 8:2 similarly calls the heavenly sanctuary the 'true (alethinos) tabernacle' of which the Mosaic one was a copy.