It's true that the way they have defined it in the split license, you are not using imagesharp as a "transitive package dependency" if you copy the source code from the git repository of an apache licensed open source project rather than using the object code.
However, that's irrelevant because they are licensing it to the open source project under the apache license.
Once the open source project has it under the apache license, you can copy it from the open source project under the apache license, because the apache license allows that, and the split license won't apply in the first place.
The linked page even says "Once granted, You must reference the granted license only in all documentation," so if they have the source code in their repository it will literally just say that it's apache licensed with zero mention of the split license by the explicit instructions of the creators of imagesharp.
If they say that's NOT the case, because they are imposing additional restrictions, then they aren't actually licensing it to the open source project under the apache license in the first place.
Otherwise, they realize that this is a massive loophole but are still hoping that companies will just pay up. I think this is what's actually the case (and why the hoped the .net foundation would accept it), but they don't want to come out and say that explicitly because then it would defeat the point.
It doesn't matter, because these clauses are only relevant when determining the license granted to someone who is consuming the library. If an open source project does that, the following item applies:
"Works in Source or Object form are licensed to You under the Apache License, Version 2.0 if ... you are consuming the Work in for use in software licensed under an Open Source or Source Available license."
So at that point the author of said open source project gets to redistribute the source of the library under AL - and only AL defines what further derived works can and cannot do with it.
It does in a court of law. A defendant would likely need to demonstrate good faith that they're using the source code of the project from which they acquired the license to ImageSharp. What matters is a judges interpretation, and intent matters.
