I'm sorry if this is a dumb question. Recently I was learning that it's impossible for us to measure the speed of light in one direction. We can only measure it in two directions and we assume that it's speed is the same in both directions by convention.
But we have been able to directly observe the effects of the finite speed of light since the 1600s, when astronomers tried to use Io as a way to measure time without a clock and found that it appeared to orbit slightly faster while the Earth was moving towards it and slightly slower while moving away from it, for a total maximum discrepancy of 16 minutes. This delay is not from light bouncing off of Io's surface, since it does so in both situations. The delay is only from the light needing to travel an extra 186 million miles between the two measurements.
So my question is, is this second example an actual measurement of the speed of light in one direction? I don't see why one source says we can't measure light in one direction while the second source suggests that we can. Please help me clear up this contradiction.