because it said that a squirrel could survive, and said something about the distance of the legs, that part is very unclear to me
Every electric conductor has a property called "resistance." You may have heard that "electricity always follows the path of least resistance." That's almost true, but really, its only most of the current that follows the path of least resistance.
When there's more than one path for current to follow, the fraction of current that follows each path is inversely proportional to the resistance of the path. The path with less resistance gets more current and vice versa.
The resistance of an overhead power line is extremely low compared to the resistance of the body of a squirrel. -Especially if you compare it to the resistance of a squirrel's dry skin. When a squirrel walks along a line that is carrying current on a dry, sunny day; virtually all of the current will stay in the line, and only a tiny, tiny amount will go through the squirrel's body.
You can ignore the part about the squirrel touching the tree and the power line at the same time. That's a case where the squirrel's body would be the only path for current to flow. That is unlike what happened to the caribou.
What happened to the caribou was that lightning sent a huge amount of current (could have been as much as 100,000 Amperes at the point of the strike) travelling along the surface of the ground. The caribou were wet. And the resistance of the path through their wet skin and their bodies probably was not very different from the resistance of the ground that they were standing on. Therefore, a significant amount of current took the detour, up one leg, through the animal's heart, and down another leg instead of staying in the ground.
Like I said, a big lightning bolt can carry more than a hundred thousand Amperes. It only takes a few hundredths of an Ampere to put an unlucky mammal's heart into fibrillation.
P.S., Sqirrels in front of my house usually walk on the telephone wires and TV cable which are closer to the ground than the power lines. Those wires are low voltage, carry very little current, and they're insulated (i.e., covered in very high-resistance material).