Not quite sure, you'd have to ask the Asuswrt-Merlin addon devs who worked with Cake as to how they could determine that it wasn't working properly with NAT acceleration enabled.How would I test for that? SQM is definitely working as I can cap my speeds and it sorts out a lot of the bufferbloat according to dslreports
It's also possible that Marvel does not do flow caching but only employs other techniques to speed up NAT throughput. I'm not familiar with their architecture.