Without knowing more details about the battery, "20Ah" alone does not convey enough information to determine how long the battery could power a given load for. If I need to power a 100 watt lightbulb, will a 20Ah battery power it for an hour? 10 hours? 10 days? No way to know.
Wh is the unit of stored energy, Wh is what I want to see. Even the official Amazon product page for it doesn't list a Wh figure.
Also, in this particular instance, phone batteries are measured in miliamp hours, so it makes the thing I actually want to know, how many times can it charge my cell phone, really easy to figure out.
But as somebody who tinkers with inverters and such, I agree, it is annoying. It is still generally not that hard to do in my head, and trivial with a calculator. But I’m with you.