Do you understand the gulf between prokaryotes and eukaryotes? I have explained the evolutionary link between the two on a previous thread. The topic of this one is ubiquitous proteins in eukaryotes.
Where does it say specifically in the O.P. that this thread is only about eukaryotes?
Nowhere.
Instead, we read the phrases:
”Some proteins are so vital to life that they are shared by every living thing.These are coded by bits of genetic code that are called ubiquitous genes. An example of such a gene is Cytochrome C . . .”
“If all living things evolved from a common ancestor then we can make a prediction about the differences between the Cytochrome C protein in different species.”
Now you object that the thread is not necessarily about every living thing. - And the fact that bacteria are living things with Cytochrome C is somehow ‘off the table’ —since they aren’t eukaryotes!
What a poster you are!