I believe they assume that sdists always have the same requirements, so you can build locally to get the requirements once and then builds on other platforms will come out the same.
And for environment markers, IIUC the idea is that they try to be conservative and in the lock file record “all the versions that might be relevant, regardless of whether the markers are true or false”. Then when installing on a particular platform, they do another run of their resolution algorithm while taking markers into account, but restricted to just the versions that appear in the lock file.
What I don’t know is how the “all the versions that might be relevant” part is done. I’m not aware of any simple correct algorithm for that, and the brute force approach requires solving an exponential number of NP-complete problems, some of which might fail, but you don’t even know whether the failures are relevant until you try to use the lockfile later. I don’t think they do this, but idk.