I do not have a definite answer, but this article: https://en.wikipedia.org/wiki/Sea_level_rise explains that sea level is measured by satellites in Earth orbit. It is not explicitly stated, but I presume the deviations in this orbit from circular are known and thus such a measurement fixes the height of the sea relative to the centre of the Earth. This, after all, is the one fixed point in the system we can rely on. According to my information the rate of rise of the Himalayas (relative to the centre of the Earth, I presume) is >1cm/yr and the mean rate of rise of the sea is ~3mm/yr. But you are right. The articles I have seen on the web do not explicitly state what the rates of rise are measured relative to, so it is bit frustrating. Perhaps if a geologist reads this thread, he or she can confirm or explain further. Post 18 from Iceaura suggests it is not that simple.