It's not really about sparing them - it's about him not making himself needlessly vulnerable by delving with extreme graphic detail into a horrifying, traumatic experience he had. Emet is demonstrably a very emotionally defensive sort at the same time he's trying to appeal to the Scions, which is what makes a lot of his behavior throughout Shadowbringers so interesting because of the resulting self-sabotage. But yeah, getting into debates over the precise nuances of Emet-Selch's characterization and his internal war with himself, as far as this context goes, is almost certainly a non-starter (I have been down this road before, lol!) It just comes down to "Do we take his direct POV as written in third-person or his dialogue simplifying things to an outside party as more reliable, if we're going with the 'he's unreliable' slant to justify dismissing one or the other'?" and how inherently silly it is to use a take by Emet-Selch to dismiss... a different take written around Emet-Selch.
We know he's partially inaccurate in the Crystarium explanation. We know his demonstration never reflected the full scope of the physical reality of the Sundering as far back as 5.0 itself. People can take the interpretation they like, but I dunno, I'm more inclined to go with the one that hasn't already been demonstrated beyond doubt to be inaccurate, the one reflected in direct description and not second-hand dialogue, and the one written at least two years more recently to show events that, as of the writing of 5.0, the writers have fully admitted they didn't have the details or background fully planned out or in mind.Either we accuse him of being inaccurate in one of those two passages or we accuse him of it in the other. All points considered, I think inaccuracy in the non-FFXIV story to be more likely.
Let me put it this way. An FFXIV fan decides to try out the Nier crossover because they heard, as advertised, it's about Emet-Selch and it's written by Ishikawa herself. When they read it, are they going to accept what it shows as perhaps an updated portrayal of the Sundering, or go into convolutions about how it doesn't precisely match up with a piece of exposition from 2+ years ago, so clearly this means that what Ishikawa has written is just Emet being nonsensical, and what a waste of time this all was, just shows more of Emet being overdramatic about what happened, etc, etc?
Do you really think the latter take is what Ishikawa had in mind when she personally wrote this story after Endwalker?