Well, we already know that the Isûphiryas was filled with lies by the Nonmen who wrote it. We might stretch this to say more so, filled with half-truths, incomplete stories and allusions that are not quite the whole story.
Then that is given to Seswatha, who we know was a definite liar, a clear manipulator, and all-around shady character, as the only remaining copy? Seems dubious he wouldn't change anything, especially anything that didn't support his plan. My hope is that one day, we'll get a book where we learn about what the real, original Ihrimsû version said. I have little doubt there is still one in Ishterebinth, but no one probably reads it because it's too depressing.
The Saga are also not totally factual, not as the Madate knows it, and yet, the Mandate are being lied to by Seswatha as well, so who knows how far off they really are.
Remember that the Tusk is also filled with lies. The Inchoroi made it to manipulate the Ketyai to invade Earwa and kill Nonmen.