I believe that's correct. They were formally recognized as martyrs, as such they gave the 'supreme witness', united with Christ:
2473 Martyrdom is the supreme witness given to the truth of the faith: it means bearing witness even unto death. The martyr bears witness to Christ who died and rose, to whom he is united by charity. He bears witness to the truth of the faith and of Christian doctrine. He endures death through an act of fortitude.
I believe the Anglican bishop was held for a month before he was executed. He had plenty of time to renounce his faith. I think they all had opportunities to renounce their faith.
(my $.02) - the pope can't offically canonize the Anglican martyrs of Uganda because that would be like stealing them from the Anglican communion. But if/when the Anglicans return to full communion with the Catholic church they and some others would be proclaimed capital "S" Saints.
Blessed Pentecost to you.