Was Coach Hong Myung-bo's tactical choice appropriate?

Did Coach Hong Myung-bo fail to properly assess the dynamics and readiness of the team? I subscribed to watch the Korean national football team's World Cup matches, but I am greatly disappointed with the outcome. Coach Hong publicly mentioned a tactical decision to bring Son Heung-min in during the second half, but it seemed that the other players were not prepared to support this. I had no idea how the situation had come to this until a reporter asked during the press conference if the players had perhaps suffered from food poisoning. Was it a problem to use unprepared players as tactical expendables against the Republic of South Africa? As a coach, why didn’t he check whether the players were ready for such tactics and had confidence? I want to conclude my personal thoughts for the next World Cup through this question. I am curious about your opinions.