What does this mean: 更不是被告?
"not the defendant" is just "不是被告"

"let alone the defendant" in Chinese is "更不必提被告"  or "更不用說被告"

"更不是被告" means "specifically not the defendant"

Example:

他不是被告 = he is not the defendant

次被告也判死刑, 更不必提被告 = even the second defendant is sentenced to death, not to mention (let alone) the defendant. -- of course he has to be sentenced to death

犯案的不是陳某或張某, 更不是被告 = The one who committed the crime is not Chen or Zhang, certainly (specifically) not the defendant.