You still don't get it, do you? Updatable non-free firmware *ensures* the hardware behaves in a known way by letting *you* check that the damn firmware file is what you expect it to be. If firmware is invisible and hidden then *you* can't check in any way that it is correct.
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RYF says the main CPU cannot be loading the firmware. In most cases that implies the firmware is also not readable by it, because it has to be stored in out-of-band read-only memory and loaded by some external mechanism.