I suppose it hinges on democratic legitimacy. Taiwan obviously is supported (as a de facto state entity) by more or less the whole population. Is the same true for the D/LPRs? Do we think a majority wanted to then join Russia, and that the referendums were free and fair (especially given the 'results' in the Kherson and Kharkhiv regions which did not support Russia).
I don't know the answer to that question, but that is what hinges on whether one supports the existence of the D/LPRs as independent entities. Whether they are truly the reflection of their people's right to self-determination and whether they, as pseudo-states, actually have democratic legitimacy.