Well my hypothesis is already in the title here's some supporting points.
He fits the physical description perfectly. This is theoretically possible to enhance with hemalurgy and if anyone could pull it off its Ruin (though admittedly it might have just been easier to wait for a appropriate child).
His personality and behavior was perfect as well, must harder to create unless he is hemalurically influenced.
And finally, he is a Seeker powerful enough to sense the Well. The only person who has been demonstrated to have that skill is Vin. Who can burn bronze naturally and has a power boost from her spike.
The main counterargument is that Ruin adjusted the prophecies to fit Alendi just as he did Vin. Kwaan even tells us that Ruin did alter them. But Alendi was still close enough to the mark the Kwaan with his uncorrupted memory saw him as a match.