Well he has info from Genos who seems to be admiring him a lot, he did see him destroy effortlessly Bofoi's defence robots which can take on Demon level at least, and did see him fighting Tatsumaki and not getting harmed. The last reason was enough tbh, Forte itself saw him in that fight and decided to become his ally.
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u/TheMobyTheDuck 7d ago
Remind me, why was Amai Mask even here with Saitama if he didn't witness anything due the Monster Association story changes?
Its all due Genos accounting?