@
Nanya spoiler tag that, it's a little big.
It's not apt to discuss about Vivio and 'how strong was Olivie' here. It's all very circumstantial.
As far as we know, her being the strongest could just very well refer to the whole Sankt Kaiser's rule, rather than just individual strength. Surely she must have had defeated and brought several other Belkan Kings under her wing, have several Lost Logia class weapons, and her own knights/army.
The sum of 'Belka's Sankt Kaiser' rather than the sum of Olivie. Besides, being a pure 'Saint King' didn't exactly make Vivio a super unbeatable 10 years old genius mage, since apparently Rio is better than her... and let's not get into comparisons with the likes of Nanoha/Fate at 9. Even Erio at 10 seems stronger.