It became a challenge when it became a challenge?
Hmm
And the studs didn't fly into his shin
The Ukraine guy ran into the foot that had just cleared the ball
It wasn't a tackle or the Sweden guy challenging for the ball, he had already won it and cleared it.
The Ukraine players momentum took him into the Swedish guy making it a challenge
Yep, the point is the ref cares that two players were engaged in a challenge (which they may define a little more widely than you appear to) and one of them conducted himself dangerously. People can try to analyse the minutiae and find justification but that is how the rule is, there are only really two parts for the ref to analyse.
It says nothing about who gets there first, who has momentum, who sees who - all irrelevancies, you can distract yourself with them but it is redundant in terms of the rules. Of course you can have an opinion on what they should be, but it doesn't mean the ref erred in applying them on the night.
The refs seem to define dangerous play as lunging, high, studs up, we have seen this time and time, I'm not sure if this is fully outlined but tonight would support the notion. As a player once your body action falls within what the refs seem to interpret as dangerous then all you need to throw in is a challenge and you are on your way.