Isaiah foretold the manner in which Jesus would die – something that did not exist yet. The detail of what he would take on and how was so precise, it couldn’t be fulfilled by accident.
By Christ fulfilling the manner of death, it also established that resurrection had to be coming as well for the entirety of scripture to be fulfilled.
Isaiah wrote about one who would come and die for the guilt of others. What Isaiah wrote about was not specific, but vague in the eyes of people before the resurrection. After the resurrection this prophesy would have been hard to ignore.
Isaiah spoke of someone who would come and be tortured and killed for the sins of many. Jesus fulfilled these prophesies in the manner of how he was tortured and crucified. This prophesy would have been overlooked at the time of Jesus, but would have been made clear after his death and resurrection.