What is the order of this traversal method?
This is a depth first recursive binary tree traversal.

Such a traversal may be qualifed as pre-order, in-order, or post-order
if you do some specific action on each node, before the first
recursive call, between the two recursive calls, or after the second
recursive call.

Actually, you can have several actions to perform, some in pre-order,
some in in-order, and some in post-order.

In your case, you have to collect and combine the information from the
two subtrees, and that is necessarily in post-order. But it is just
using terminology, which I see as moderately meaningful in this context.

These concept often suppose an order of the daughters of a node, but this is largely irrelevant here. You can process your subtrees in any order, left first or right first. I guess that is also why I am a bit reluctant to make this postorder distinction. The pre-order, in-order, post-order distinction assumes you are exploring depth first from left to right.

They also correspond to distinct linear notations for representing binary trees, where nodes are labeled differently from leaves: prefix, infix and postfix.