That's interesting, I'll have to read up on that. This limit is for purely thermal engines that run on a hot and cold heat transfer cycle. So I can imagine something hybrid would do better, but I'd be willing to bet that if we took a close enough look at that engine we'd find the thermal only part of the engine being under 50% efficient, and the rest was boosted by a different system.

The maximum efficiency of an ideal combustion engine (Carnot cycle) is only 50%. This is a theoretical limit, no real engine can actually reach this efficiency. The fact that we have engines in the 30% range is already pretty impressive.

Why aren't the improvements bigger? Well, because the closer you get to that limit the harder it is to get closer to it. In other words, it takes increasingly bigger technological and engineering advances to make increasingly smaller improvements to efficiency as you approach that limit.