Hi gb

Firstly, the bulk hydrogen loss was aeons ago. What we measure now is the deuterium ratio left behind, which is quite high. Thus it's reasonable to infer Venus once had a lot more light hydrogen combined with water.

Secondly, the solar wind has been observed blowing away Venus atmosphere, so back when it was losing bucket loads of hydrogen it would've been quite a plume. Now the hydrogen loss rate is much lower and most observed gas loss is broken bits of CO2.

Thirdly, there's nowhere to hide the ocean in the observed atmosphere. Venus really is very, very dry. There's probably not much left in the mantle or the rocks either, but that'll take a lot more work to quantify.