First time reading the manga... Something doesn't sit right with Mello?

Hi, sorry for the vague title but I did not want to spoil. I just finished reading the manga, so sorry if this question has been asked, but there's something that doesn't sit right with me regarding the Mello x Takada event at the church.

It's been stated many times in the Death Note rules, as well as in the Kira case, that if the circumstances of the death lead to somebody else dying, then the person will only die of heart attack regardless of what the person wrote.

This is exactly what happened with Takada, no? Light wrote that she would die of suicide by setting everything on fire. This would have worked if she was alone, but Mello was also in the church and died from the fire. This is definitely a case of a collateral victim's life being shortened from the Death Note, so Takada should have died of heart attack.

Am I missing something here?