I probably wouldn't disagree, if we're speaking about a top-ten based on overall record, but there are plenty of Champion Hurdle winners in that period that he was better than - measured either on career performances or his CH win, imho. Off the top of my head, I'd be thinking Flakey Dove, Collier Bay, Make A Stand, Hors La Loi III, Brave Inca, Sublimity, Katchit, Punjabi, Binocular, Rock On Ruby, Espoir D'Allen, Epatante.
Rooster Booster was campaigned differently from your usual Champion Hurdle type horse. He was somewhat of a late developer, and took some figuring-out as to his optimum conditions (undoubtedly helped by the switch to Hobbs), but I don't think that necessarily drags him down, tbh. Nobody has claimed he was a true great - just that he was a top-class animal on his day, in the admittedly relatively-brief window when he was operating at full capacity.