I wrote a paper (P1) which went through peer review and got published. The paper introduced a new graphical user interface (GUI) primitive.
I then wrote P2, which extends P1, turning it into a new class of UI, which had no analogs at the time of writing (and maybe still doesn't, but the window of opportunity is shrinking).
P2 is not just a thin layer on top of P1, it comes with substantial enhancements that are the result of our "lessons learned from P1" analysis + new ideas.
P2 stands on its own, and it refers to P1 as if it were written by other people, per conference requirements. P2 also provides a short intro about P1, such that someone unfamiliar with P1 can still get the picture. It also provides comparisons to analogs of P1, explaining how our approach differs and arguing that our approach is better.
The set of P1 analogs consists of ~5 papers. One is a preprint, 3 are just providing their designs but not including any usability evaluation details at all, or deferring that to future work. This leaves us with one good alternative, which we were not aware of at the time work on P2 started. However, we did refer to this alternative in our related work section and compared it to our own product (there are some ideological differences in our approaches).
P2 explains that we chose to base our work on P1, because at the time of writing P1 was the only artifact of its kind that has gone through formal usability tests and provides detailed results of their evaluation process. Of course, the fundamental underlying reason is that P2 is the continuation of our previous work on P1.
P2 has been submitted for peer review, and a reviewer rejected it, one of the reasons being "it only considers one paper [referring to P1] containing a very similar design".
What is the right way to explain why P2 is built on top of P1 without compromising anonymity?
p.s. note that the reviewer in question seems to have forgotten about the section where we compare P1 to alternatives. It could be that that section is not convincing enough, not well-written - but in that case the reviewer would have said "it is not convincing enough" rather than "only considers one paper". The review is otherwise thorough, I cannot say that I have the impression the reviewer hasn't read the paper fully.