You've got most of it. Relative to the sensor, it's not misleading to specify a maximum aperture of f/xyz.
Focal lengths and apertures are constant regardless of the sensor size e.g. a 300mm /f2.8 lens on my 70D is still 300mm at f/2.8 on my friend's 5D Mark III despite my camera having an APS-C sensor while my friend's is full-frame. It's just that the crop factor gives the appearance that the focal length on my 70D is ~480mm whilst the reduced amount of light getting to with a smaller sensor gives an aperture equivalent of f/4.5.
It's absolutely fine to specify the f/2.8 aperture, you just have to bear in mind that you will not get the same performance in terms of depth of field or the amount of light coming in when you compare it to a camera with a larger sensor. To get the equivalent light and DoF as an f/2.8 lens on a full-frame sensor with an RX10 would need an aperture of ~f/1.0 so as you've rightly worked out, one of the implications with a smaller sensor is that it's more difficult to get a shallow depth of field.
DSLRs: Canon EOS 7D Mark II, Canon EOS 70D
CSC: Canon EOS M3
Lenses: Canon EF-S 15-85mm f/3.5-5.6 IS USM, EF 50mm f/1.8 II, EF 100-400mm f/4.5-5.6L IS USM, Canon EF-M 18-55mm f/3.5-5.6 IS STM, Canon EF-M 22mm f/2.0 STM