IMO the author is a little over-claiming this work by naming 'recursive'. Quote from this blog:

Lastly, in our experiments we only consider a recursive depth of 1 — i.e. the root LM can only call LMs, not other RLMs.

but we felt that for most modern “long context” benchmarks, a recursive depth of 1 was sufficient to handle most problems.

I don't think a size 2 call stack algorithm should be regarded as 'recursive'.