The "S" in "SRE" is pronounced "ess" which would take an "an" as the article. The title is grammatically correct. You would also say "an FBI agent" or sending "an SOS message."

No, "an SRE" is correct here; it's actually "a SRE" that would be bad grammar.

Whether you use "a" or "an" depends on whether the next word starts with a vowel sound, not with a vowel letter; since the letter "S" is pronounced "ess" in an acronym, you would write "an SRE," but "a Site Reliability Engineer."

It's an initialism, not an acronym. And _an_ SRE is correct.