I'm not sure what you're getting at. Matazo came off the bench in his first appearance for us too, that didn't mean he was injured.
My point is Alzate was fit and fine at Liege, was released by Brighton, trained with Ipswich and was not offered a deal, then signed for us before badly injuring himself, and is now playing well. Ipswich had a good look at him as you said and decided not to sign him.
A quick Google shows he had a minor knee injury at Liege in December of 2023 but was otherwise fit for them. So fitness was not a necessary context for why Ipswich didn't give him a deal.