The Swaziland and Mbabane Swallows attackers were on trial at Phunya Sele Sele looking to win the favour of head coach Serame Lesoaka.
However, they failed to earn contracts with the Absa Premiership club.
"They are good players, no doubt about that," Celtic CEO Khumbulani Konco tells this website.
"Both players did well but they are not what we are looking for in terms of our striking force."
Siwelele were recently beaten to the services of Golden Boot holder Collins Mbesuma by Highlands Park, while Letsoaka on Saturday downplayed talk of a move for Orlando Pirates hitman Lehlohonolo Majoro.