Yes, they even used TLABs. I believe the issue was that Xamarin was more interested in mobile performance at the time when SGEN was seeing heavy development and

Yes, they even used TLABs. I believe the issue was that Xamarin was more interested in mobile performance at the time when SGEN was seeing heavy development and preparing for release, so they optimized for memory usage instead of throughput. The generational part was probably more of a hindrance than a benefit at that point in the process.