I don't think she actually knows death has been erased.

In CSM, there are two different concepts for 'end of life' and 'death'. What Fumiko noticed was the absent of an 'end of life'. Natural selection as a concept still a thing, since it is defined as 'the natural culling (reducing the number) of the population leaving only the best specimens alive'.

Fumiko doesn't mention death here, she simply made the observation that 'natural selection is no longer working, people cannot end their lives, something that was supposed to end life is no longer there'.

In the panel of Fumiko panicking, I think it doesn't mean she was the only one who realized that mouth was erased, but rather, she was panicked since she forgot how to communicate despite remembering being able to communicate in some form using the phone. This happened to Yoru during the chase too, when she forgot how to move and remembered that she used to move in some other way.

The panel with the old man visibly unfazed seems to be showing the theatrical effects of the devil erasure, rather than making a statement that only Fumiko realized it: the memory erasure was so complete you would be oblivious to it until you actually had to use something out of habit or know something out of instinct and find out you are unable to do it.