Yes, but I think the solutions work fine; tap, hard press, whatever.

Don't get me wrong, I share the annoyance with interfaces where I have no way to find the actual timestamp. It's better to only have a timestamp than to only have the "human readable" version with no way to get the timestamp. But (in my opinion) the best is to have the human readable version, with a way to get at the timestamp.

I think the problem is that the mechanism to trigger mouse-hover is not clear on a touchscreen - there are several different ways that need to be remembered and tried, as evidenced by your comment.

I wonder if a solution on touch is a (?) icon next to the human date? Or is that too much clutter?

I like ? icons that can be either hovered or clicked on to reveal a popup with additional info.

Yes, but I think the venn diagram of people who really want to see a timestamp and people who are willing and able try a few different things to find it is pretty close to being a circle.