Considering Goku’s condition during his fight with Freezer, we can explore a different angle. When Goku transforms into SSJ, he is exhausted from using KKx20 and injured mainly due to the beating he received at the hands of Freezer while gathering energy for the Genki Dama. It’s possible that Toriyama referred to a 10-fold increase from Goku's state at that moment, rather than his full, fresh power.
In the moments leading up to the transformation, we can see Goku in several panels with glowing eyes. This might indicate that, despite being badly injured, he had reactivated the KK as it seemed to be activated just at the moment he launched the Genki Dama at Freezer.
In the interview we are referring to, Toriyama wouldn't be explicitly saying that Goku directly multiplied the power he had at that moment by x10, but rather he would only be making a comparison between his power before transforming (with KK activated) and his power once transformed. In other words, he wouldn't be saying "Goku multiplied his base power by x10," but "Goku at instant X was 10 times stronger than at instant Y." Toriyama would simply be making a comparison between Goku's power just before transforming and after transforming, leaving the actual multiplier with respect to his base power unknown.
If we were to speculate about this multiplier, we could assume that due to adrenaline or some other cause, Goku regained some power since he no longer appears so tired, even when he detransforms before Freezer ends up cut in half, indicating some recovery or stabilization of power after the transformation. That said, it’s even possible that Goku experienced a power increase due to Zenkai Power.
Here is a speculative breakdown of the power levels during the fight:
Initial Goku: 3,000,000
Initial Goku KKx10: 30,000,000
Initial Goku KKx20: 60,000,000
Freezer 50%: 60,000,000
Injured Goku KKx10: 9,000,000
Goku SSJ: 90,000,000
Injured Freezer 100%: 90,000,000
Base Goku at the end of the fight: unknown
Thoughts?