If the only defending unit is killed during a move trigger from the attacker, is the pending showdown still a combat showdown?

No. If the defending unit is killed during the move trigger before Cleanup, the outcome is a non-combat showdown, not a combat showdown.

Here's why: when a move makes a battlefield contested, the contested status is applied immediately (Rule 445). But Combat isn't staged until the next Cleanup (Rule 323.9), which happens after the entire chain resolves. By that point the defending unit is already in the trash. Since Rule 323.9 only stages a Combat at battlefields that "have Units present controlled by opposing players," and the defender's unit is gone, no Combat is staged — only a Showdown is staged (Rule 323.8). So when the turn player later initiates that showdown (Rule 323.11), it opens as a non-combat showdown, not a combat showdown.