That statement is hard to reconcile with the fact that the phone is allegedly compliant with, and is being submitted for, RYF certification¹, which it would never qualify for if the phone has binary blobs.
1. “And we are compliant with, and submitting for, the “Respects Your Freedom” certification from the Free Software Foundation.” — https://puri.sm/posts/librem-5-shipping-announcement/
For example most CPU's are written in something like verilog or vhdl and include internal ram on the chip and include a prom with a little bit of code that handles start up tasks.
From what I can see Librem 5 is a general purpose computer with 2 sealed black box modem modules.
Yes, there can be any level of proprietary evil inside those seal boxes... and inside your disk drive controller of your PC.
The question then really is can that proprietary evil control the functions of your computer.
With the disk drive controller, probably pretty hard, with intel ME, probably pretty easy, with the Librem modems? My guess is probably pretty hard.
Modem can record audio or location and send it out on remote request when it's powered on. When modem is used to access internet it can add JS code to html pages and execute code that way.
Touch controller can record touches that look like pin entry (it can observe touches, and make some guesses about frequently repeated touch patterns after powerup/wakeup) and replay them after some secret swipe gesture. If UI patterns are known, touch controller can probably tap information out to a web page somewhere and hit submit.
Just because there's no direct access to memory, doesn't mean even these "sealed black boxes" can't affect/use software running on the main CPU via other channels.
Also if the drivers are not written in a manner that they consider devices they control hostile, I would be surprised if modem would not be able to return specially crafted/unexpected messages over USB that would allow for arbitrary code execution in the kernel or in userland.
That said, Librem5 is still ahead compared to any Android phone regarding freedom and especially privacy part.
I spent my time in Purism cleaning such false claims about laptops and I was open about where we stand with it. That said, laptops have less issues than modern phones regarding freedom part and people just need to be aware and honest about it and not say "no binary blobs" when actually you will use binary blobs for crucial capabilities of modern phone otherwise you will have a brick in your pocket.