I haven't read Saito's books, or looked too deeply into degrowth as a movement. I just read this article and thought it made some good arguments against what it claims are Saito's understandings of Marx. I'm not sure I agree with everything, but I thought it was interesting enough to share.
Degrowth is essentially a strategy of cutting back on energy inputs in order to prevent catastrophic climate change. It simply described a possible pathway to stay below 1.5°C of warming. I use the past tense because we already passed that threshold unfortunately. What you're calling degrowth is instead that catastrophic climate change will destroy the material basis for our current modern civilization, thus reducing energy inputs (and presumably ending further anthropogenic warming).
So, I think these are quite different. Strategy to avoid apocalypse vs result of apocalypse
Intentional degrowth on terms we make ourselves, versus sudden emergent degrowth from critical existence failure.