there's two kinds of "judging history by modern standards",

there's using it as equalizing bleach, 'people of their time', 'i can worship a mass murderer if i want to'---this is navelgazing and ignorant.

and there's trying not to take our 'baggage' with us when we study the past. 'morals' encompasses a lot more than murder and slavery being bad and it's important to try and perceive things as much as practicable in the ways they were in the past. most dramatically and acutely this is reflected in sexuality--the whole 'historian says they were just friends' joke is the result of taking a modern (victorian) moral standard into the past, and misinterpreting evidence around that. people can also do this from a good moral analysis, iirc it was st augustine(?) that had a lot of writing about the importance of sex within marriage--and the good feminist critique of that was interpreting it as cementing patriarchy in christianity, controlling women's bodies. but the particulars of the terminology used has more recently cast doubts on that interpretation, was it actually about telling men not to patronize brothels and their unmarried, -enslaved- workers? that's a pretty dramatically different read we couldn't make if we took our modern prejudices against christianity & marriage into the past but goes some way to explain why early christianity was so popular with women and slaves.

the lazy excusal version of 'judging history' is a pretty typical case of a complicated concept in a field being hazily remembered & used as a rhetorical bludgeon---with a little help from some bad faith professionals using it cynically, of course.