In regards to the question, I believe there are many elements that one has to factor in.
As I'm not necessarily equipped to delve into the specifics, I do happen to have strong Leo influence within my alignments and can offer that particular insight.
As for myself, power & strength are intrinsic elements of our nature. As we may know this & embody it, we do not have the need/desire to act upon it. To prove it. We know it. We are the embodiment of it. We also live on principle. First & foremost, we have heart. The heart of the lion. This, I believe to be the major differentiating factor.
A "Powerful" Scorpio may be cruel, callous, and calculating. May hide, weaken their opponents- physically/emotionally/mentally - even have others do their bidding, then strike at the lowest possible state a being can be at. Claim "strength" & "power".
While a Leo probably doesn't even acknowledge such an "opponent". It doesn't exist. They are not dependent on external sources: people, weaponry, etc. They do not need "validation" of any kind. A Leo could have just about every odd stacked against them & still annihilate & dominate. They know it. They do not have to prove it. Nor do most have the desire to. Again, the heart of the lion. Principles. Morals. Ethics.
A "Strong" Scorpion may shatter at a displeasing word / harsh criticism spoken
/ uttered to them. May break under the slightest "hardship" or "pressure" bestowed upon them.
The strength of a Leo can & will withstand the stuggles, obstacles, and challenges of 100 lifetimes. They know the majority would break under the pressure of one hardship they may have encountered in their lifetime. Let alone 100 lifetimes.
There seems to be a difference between one who actually has it & one who desires it.
Also, our society seems to have an extremely perverse understanding of what power & strength really are. Like everything else.
Again, this a very base application.
I would actually argue Leo might be the weakest sign in the zodiac. Just because they know they can handle anything, doesn't mean they actually can. There's evidence enough for that in the fact that their go-to method of coping is denial and willful ignorance. Ignoring the problem or person. Like you said, that person doesn't exist to them. But the fact is, a person with a healthy strength of character doesn't need anyone who insults or challenges them to disappear. The fact that they don't exist to them is proof just how badly Leo
does need validation. Without the right mix of supporting features, Leo on its own lacks the fortitude to face harsh realities, especially about themselves. This method of coping speaks volumes.
They also wouldn't need to be so self-aggrandizing if their abilities could speak for themselves. This is in itself is a form of coping.
It's easy for people to get wrapped up in Leo's vivacious self promotion, but ask enough questions, ask them to show you what makes them so great, and the circus they put on starts unravelling fast. I'm sure I'm not the only one here who's witnessed their share of Leos tantruming over a minor or even imagined slight to their ego.
In relation to Scorpio, while I would call Scorpio stronger, more powerful, it's not a healthy power. Not for them or those subjected to it. But ultimately, however toxic their coping, Scorpio can take a hit.
Because of the life I've led, I've known many, many people who come from abusive homes. The Leos, basically without exception, broke. They turned inward, they became too afraid of people to put themselves out there in any way. The abused Scorpios may have done some questionable things along the way (and still now), but they held it together. The are intractably themselves, and all the weathering the world could throw at them can't stop that force.