I mean, in principle it is possible to use the parasitics as a matching network so that 51 ohms ends up close to 50+0j ohms at high frequency. But even if you managed to pull that off, it would be quite sensitive, so you might well end up having to fine-tune every load by hand. At 3GHz just building something that has lower parasitics in the first place really looks like a much simpler option.
But, it turns out I actually measured the supplied load when I experimented with the NanoVNA v2 (without +) a few months ago. So let's look at some data. The reference was my working load standard, which was selected from a bag of Huber&Suhner 18GHz SMA loads. So not ideal, but should be good enough to get a qualitative picture (incidentally, that one measures almost exactly 50.00ohms DCR on my 34401A).
There is always a chance they changed suppliers in the mean-time, so yours may differ. In any case, at low frequencies, it is 50.8 Ohms, so seems to be 51 Ohm as well. Overall performance is however dominated by the reactive part. To get an idea if the 51 Ohm are actually beneficial, I fit a model consisting of a piece of transmission line, terminated by R || C. Now we can compare to, say, R=49.9 Ohm: the real part of the impedance is indeed closer to 50 Ohm at higher frequencies, but that is not really helpful as the return loss is still worse. Needless to say, the low frequency performance is degraded with 51 Ohm.