Me, too. Of course you're right, as proven by the only ancient instance of this verb (λησμονέω, not in LSJ) in John of Damascus (ca. 700 CE), Parallela 95.1513.47: Ποῦ οἱ πλουτήσαντες πρὸ ἡμῶν; ἐλησμονήθησαν ("Where are those who've enriched themselves before our time? They've been forgotten!").
"Λησμονηθέντες illimati" is another LSJ Heraeus-mirage ("Gloss."), this time with a well-deserved "dub." in parentheses. Best ignored, I think. Note that Heraeus' nonsensical illimati (lit. "un-covered with mud") is nominative plural masculine, denoting humans.