SQL Server ships with almost it's own OS implementation, aptly named SQLOS. If I had to guess, the interface between SQLOS and Windows is relatively thin (and just IO, Threads, and Memory Management).

There are a lot of other bits, such as Integrated Security Security, Windows Perf Counters, Windows Event Log, that are probably stripped out.

But I also suspect that that the Linux implementation is based on the implementation that they use for Azure SQL Databases.