Look at who else was has been undisputed champion, that alone does not make you the best of your era. It was the icing on the cake for Lennox, not the reason he was the best of his era.
You think beating Chisora, Chazz Witherspoon and AJ twice is better than dethroning Klitschko, beating Wilder 3 times, destroying Whyte, beating a prime Chisora twice etc?
Zab Judah was the last undisputed welterweight champion, do you reckon that makes him the best of his era?
As for the 2nd part, I’d never argue with anyone who said Usyk is the best technically, he is phenomenal.