Cloudflare is a MITM by design. Calling it an attack is disingenuous; you're signing up for the service of your own free will, not a victim.
If a substantiated news article came out showing that Cloudflare shared SSL keys or otherwise gave direct access to various intelligence agencies without a court order, that would essentially destroy the company. So they certainly aren't doing that.
So then the question becomes whether those nefarious three letter agencies penetrated Cloudflare with APT tools and are silently listening to everything. Our adversaries are certainly trying, China, Russia, Iran, etc. If the NSA (which lacks a mandate to act on US soil, and CF is a US company) or perhaps the FBI hacked a US company, particularly one that covers like a third of the internet like Cloudflare, that would be a truly enormous scandal.
But in the end, yes, it is a MITM. If you need your data to be E2E encrypted, don't use it.