Normally a signing would only get a two or three year deal, and a three year deal would be only at the 'bigger' club (Aberdeen, Hearts and Hibs).
Things are a bit different for players already at the club, like Scott McKenna at Aberdeen is signed through 2024, players who have 'proved' themselves.
After reading the club's press release, seems the signing came from the linkup with Atlanta United and their knowledge of South American players.