Unforutealy, your team was full-capped at the time of death. This means that the only place for the enemy to attack is at your spawn. His actions were technically acceptable in this one unique case, since your team had 0 flags. Let me know if further explanation is desired, thank you for your report & footage.
What's the point of this rule then? I wouldn't care about being killed on the base if I had another places to spawn. For me it seems a lot more reasonable to let the team with no flags get out of the base and capture at least one flag. Though I'm not sure if that's possible to technically implement, because this only happens on Paracel Storm. Maybe nuking the opponent's team would make sense, idk.