Is denying Christ under the threat of death not a mortal sin?

I heard that if martyrs denied Christ when forced under the punishment of death, the sin would not count since it was not freely chosen. Basically, they wouldn't commit a mortal sin since the mortal sin requires freedom of choice. But some theologians disagree and supposedly they would still have to repent and confess their sin. What is the Catholic perspective on this? If it's not a sin why were the martyrs willing to die if denying wouldn't be even considered rejection of their faith under this circumstance?