William Falconer's (1805) translation of this passage of the Periplus is:
"I am myself persuaded, that Achilles was a hero, if ever man was, being illustrious by his noble birth, by the beauty of his person, by the strength of his mind and understanding, by his untimely death in the flower of youth, by his being the subject of Homer's poetry, and, lastly, by the force of his love, and constancy of his friendship, insomuch that he would even die for his friends."
Henry Chotard's (1860) French translation, on the other hand is:
"Je crois en effet qu’Achille fut un héros s’il en fut jamais et mes raisons sont qu’il était d’illustre naissance, qu’il était beau, qu’il avait une âme courageuse, qu’il disparut vite du milieu des hommes, qu’il a été chanté par Homère, et qu’il a été si aimant, si dévoué a ceux qu’il aimait, qu’il a voulu mourir après les avoir perdus."
The difference, crucial to Yourcenar's text, and to Grace Frick's English translation of it, is in the passage underlined above which, in English, reads: "...so devoted to those he loved that he wanted to die after they were lost to him."