I don't know, but I think H-20 should be more strategic purpose than tactical one. If it can bring nuclear missiles inside it's body, it can penetrate Japanese and South Korean air defense platforms, and aim to Tokyo and Seoul. Also threat Manila to think twice before they agree to join the war in US side. Because with H-20 stealth capability, it can strike Tokyo, Seoul and Manila from any where outside China own territory. It can even strike Japan from eastward, strike Manila from SCS or even turn around from Russia territory / north Korea to reach Japan and South Korea from the location that they are not aware off.
With that, China can decrease the chance of Japan, S. Korea, Philippine, etc from doing dangerous adventure that can harm China intention to unite Taiwan.
And yes, I know that Dong Feng can also do the job. But think about it. Dong Feng will be very likely to launched from China mainland. And you know what, I'm sure that all US and their alliance air defense weapons will face toward China. But if China has something that bring more calculation to the table, it can change the things around East Asia.
To be clear, I'm not stating that anti-shipping will be H-20's primary role, but rather that anti shipping will probably be one of the missions that H-20 will have given to it by virtue of being a modern bomber.
For the anti ship mission, H-20s would be more important at distances beyond the first island chain rather than at the first island chain.