This is precisely why Xebec doesn’t have the motivation to kill Harald or engage in a serious fight against him. The idea of subjugating him by force was rejected by Xebec when Kaidou proposed it, which means that whatever Xebec’s plans are, defeating Harald and making him obey is not included. This also means that any confrontation between them lacks sufficient motivation for Xebec to want to win and defeat his friend.
I would need to see more — a real standoff between the two, one falling from exhaustion (an extreme difference), like Ace or Blackbeard, Lucci vs Luffy, Kaidou vs Luffy where Luffy passes out after the final blow — to determine equality here.