I didn't attend J-school, but that sounds like a questionable practice ethically. A person has a right to be anonymous, but it should be noted they are anonymous. Making up a name for someone without disclosing that fact is questionable.

Using their first name to pretend they are both anonymous and non-anonymous is worse than both endpoints of spectrum.

It is ethical and expected for a journalist to obscure facts to protect their sources. It is unethical for a journalist to fabricate facts to protect their sources.

Also as someone with experience being anonymously quoted by journalist before, it seems to be standard practice for the journalist to defer any decisions on how they would like to be referred to the source. Odds are Evelyn is the one who decided to just go by Evelyn. She probably doesn't care too much about absolute anonymity, but she also wants plausible deniability.

Owing to the fact that they said they are omitting her surname but said nothing about changing her first name.