How would an optional, official parser be toxic?

[Marxam]

How would something like that even lead to toxicity? I can't wrap my head how a personal tool that only you would see (unless you decide to do something like share it publicly) would equate to an increase in toxicity.

You just answered your own question. I'm on PS4 also and I gauge player skill based on mechanic execution and relative ease of combat flow. A dead dps will always do less dps than a decent dps who didn't die the whole fight. Anyone can smack a boss that is just standing around but not everyone can do that while dodging aoes, doing mechanics and still getting in your positional's.