Re-read it. He says that in his 5 years, wizards and specifically bladesingers are the only class that he has played past level 4 without multiclassing. He is talking about the entire class there, not specific characters. Then he mentioned a specific bladesinger he has multiclassed with that is past level 4. That's contradictory.
No it is not. To clarify:
Single class characters I have played beyond level 4:
1. I played Atelia, sister to the legendary Afelia (another player's former PC Bladesinger) to level 15 as a single class Bladesinger. Afelia as an aside was the most powerful character I have ever seen at my tables.
2. I am currently playing Naldeen an 11th level single class bladesinger.
3. I briefly played Tinta, a bladesinger to level 3, although that would not really count because she was only level 3, but I intended for her to be a single class.
#1 and #2 are the only two characters I personally have played beyond level 4 that did not multiclass prior to level 4.
Multiclass bladesingers I have played:
1. I am currently playing Taslania, the Rogue/Wizard mentioned. She started as a Rogue level 1, then she took 2 levels in Wizard (Bladesinger), followed by 3 levels in Rogue (Arcane Trickster 4) followed by 4 levels in Bladesinger to get where she is now. She was multiclassed at 2nd level.
2. I played Namia a Gloomstalker-Bladesinger (4/6) who started as a Ranger level 1, multiclassed and took 2 levels in Wizard (Bladesinger). Then three levels in Ranger to 4 (Gloomstalker), then I think 4 levels in Bladesinger to level 6, which is where she was when we stopped playing. She too multiclassed at 2nd level. I am not 100% positive about her, but I think that was the breakdown.
3. I played Ryn who started as a Monk, multiclassed to Wizard at level 2 (1/1) and then died. She was expected to become a Monk/Bladesinger at level 3 if she survived.
I have played many more multiclass characters, mostly Rogues of some sort but those Bladesingers at the top (or almost Bladesingers) I have played are the only characters I did not multiclass.