But... Does this mean that the keys of the node (in other words - matrix) are relative to its parent node?
The name specifies the bone/node which is affected by this animation channel. The keyframes are given in three separate series of values, one each for position, rotation and scaling. The transformation matrix computed from these values replaces the node's original transformation matrix at a specific time. This means all keys are absolute and not relative to the bone default pose.
I am embarrassed by the word "absolute".