Traditional BASE is even more boring and monotonous at 15-20 hours/week of low-intensity workouts. Sweet spot BASE (SSB) high-volume increases intensity a bit, and decreases time to 8-11 hours/week. And at the other volume extreme, SSB low-volume is around 3.5 hours/week and has to toss in more intensity.
The point of BASE is to build an aerobic foundation. There are proven benefits of “training slow now to get fast later” - https://www.active.com/articles/aerobic-base-training-going-slower-to-get-faster - but it takes a huge time commitment on the order of 15-20 hours a week. Sweet-spot increases intensity a bit to reduce volume, while still getting some of the benefits of traditional base. For most people, its an acceptable tradeoff.