Chapter 662 proved the statement of Kabuto...

[startup97]

Spoiler

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[johnny scars]

wait.. what?

 

[DeadManWonderLand]

Kabuto is telling him that of the current kage at that time Hiruzen was the strongest.
Not the strongest kage of konoha or the strongest kage of all time.

The tense in which he is speaking makes it evident

 

[Six Paths]

Ermmm............OK

 

[USSJ Future Trunks]

i'd like to see hiruzen dismantle atoms or lift islands

 

[Xāvî1]

What the... Never saw anything like that?