They are not well informed about these things. First of all, with two gpus on a X570 motherboard, each gpu would get x8 pcie lanes, which is more than enough for any possible scenario. Furthermore, X570 motherboards are equipped with pcie gen 4.0, which provides (theoretically) double the bandwidth that the previous generation motherboards had.
x8 pcie gen 4.0 = ~ x16 gen 3.0, and 99% of current gpus (including the 2070’s) use pcie 3.0.
You would be OK with even 3 gpus on a X570 motherboard with a x8/x4/x4 configuration (but don’t tell them that, they will freak out …).
Do a little research, if you want. Search “x8 vs x16 pcie” and read relatively new articles about this. You’ll understand what’s going on with pcie bandwidth and if a modern gpu driving a monitor of up to 4K really needs full x16 lanes to perform well.