Composition of ice giants
There is a few things one has to consider when talking composition:

It is usual to talk about relative abundances, and usually the solar composition is taken as the measure for this in the assumption that its composition is mostly prestine with a few exceptions. Now you can compare observations of different bodies and look at ratios of different materials, say nitrogen vs. carbon.

Now, materials have a different condensation temperature, and that is one of the main points of the paper. In the sun everything is in gas phase and thus has about pristine ratio.

Planets on the other hand form from solids. Thus - assuming a planet forms at distance R from the sun - it forms only from the materials which already solidify at the temperature at the given distance. (This is simplified due to radial mixing in the PPD and planetary migration, but the point is still valid). While the overall chemical composition of the PPD is the same, the chemical composition of the solids is different, the high(er)-temperature condensates are enriched, thus they are also enriched in the planets forming there

Carbon dioxide condenses from gas phase at slightly higher temperature than nitrogen. That means that you enrich the solid phase of the PPD in carbon and oxygen. Planets form from solids and only accrete gas when they have grown sufficiently - a threshold which Uranus and Neptun only barely reached. At the same time the gas of the PPD has to be in pressure eqilibrium, thus a flow towards the condensation zone exist, leading to slightly further enrichment of the condensing matter in the solid fraction - which is only partially balanced by the radial drift of the solids inward where they evaporate again.

In essence, due to different condensation temperatures, and the region the two outer gasous planets formed, they are enriched in carbon when comparing to nitrogen due to carbon dioxide ice condensing in the solar nebular. It doesn't mean that anyone took away the silica nor the water ice in the formation - as such.

I don't find there any support in the paper that the ratio of C/O can be enhanced beyond 1: the most abundant solids are slica, water ice - and the point of the paper at that distance CO which is increasingly enhanced during the formation process due to transport processes of solids and gas and condensation in the vicinity of the CO snowline (see figure 4 therein).