You do bring up an excellent point. A lot of gravel events (races, or just rides) have decent sized distances on tarmac, or really hard packed fire road. Even a S-Works Epic HT with the configuration I said will underperform there. But if one does start watching gravel events, for whatever the reason, some are mostly on gravel, with very little tarmac, and even chunks of green/easy singletrack, with bits of grass, and such thrown in. That's where the high quality, lightweight XC HT bike will perform extremely well.
I supposed it depends on how one is micro-analyzing any ride, doesn't it! KM by KM, section by section!