[__gcd]

As I understand it, fiduciary duty is a key requirement for insider trading to be considered illegal.

Insider trading law requires people with fiduciary duty to report trading, and largely allows any trading after earnings. In particular, with the view that all relevant information on the company was made public recently (if that's not the case, it's the company that would get in trouble). So insider trading is largely legal.

Also, the HK market being above the median by itself doesn't show that it's a good idea. Efficiency was defined as the relative spread in the paper. The efficiency of the HK market might be driven by other factors, like deeper adoption of technology. A better (but still not great) test would be what happened after HK made insider trading illegal.

[seanhunter]

You understand incorrectly. At least, fiduciary duty is absolutely not required to establish insider trading in the UK market abuse regime (or the EU which is similar). In the UK, simply trading on the basis of material non-public price-sensitive information is insider trading.

In the US there have been some cases which make it very hard to actually be convicted of insider trading in actual practise, but fiduciaries are not the only insiders in US securities law. Here's more details. https://www.investopedia.com/terms/i/insidertrading.asp