The question is; "How did they see Him go?". You highlighted the wrong part of v9.
They "saw" Him go as follows;
"Now when He had spoken these things, while they watched, He was taken up and a cloud received Him OUT OF THEIR SIGHT."
A cloud received Him.
The cloud received Him OUT OF THEIR SIGHT.
And now it would behoove us to now look at how Jesus said He would return;
"...and they will see the Son of Man coming on the clouds of heaven with power and great glory."
So He comes in the clouds. And He would return in the same way that He left. That leaves us with questions.
Did they "physically see" Him in the clouds when He left? The answer is "No". Not according to the verse you quoted.
So if He's going to return *just like* He left, then would they "physically see" Him returning? If we believe the verse that you quoted, then the answer is "No" to this question as well.
We can't just go halfway and ignore the second half of the very same verse (Acts 1:9), amen?
And we must interpret Scripture (Matt 24:30b) by Scripture (Acts 1:9), especially when it is dealing with the same specific subject, right?
And we should understand that the Koine Greek word behind "see" can be translated as either visual seeing, or as understanding something. Like when you say to someone, "Do you SEE what I'm saying?". SEE what I mean?
So based on all of these facts combined, the only conclusion I can draw by the Scriptures here, is that they would "understand" that He had returned, just like when they understood that God was judging Egypt, using the Assyrians in the fulfilled prophecy found in Isaiah 19:1, in which it specifically says that God "rode a cloud", yet no one "physically saw" Him riding a cloud into Egypt. Yet it says that's what He did;
"The burden against Egypt. Behold, the Lord RIDES ON A SWIFT CLOUD and will come into Egypt." - Isaiah 19:1
The bottom line here, again, interpreting Scripture by Scripture, is that when Scripture uses the term "coming/riding (in) the clouds", it is not speaking of God/Jesus up there, on a cloud, with a lasso tied to it, shouting "Woo Hoo!". <lol>
Rather, it is speaking of His presence in judgment. And in this case, Jesus specifically said that the judgment would be against the Jews.
Furthermore, if we read the entire OT, we can see all throughout it (especially the Psalms), that there are constant references to the Lord "coming down". David said this many times, in many Psalms. That the Lord had come down into the battle and stated it no differently than we see in the NT.
We must remember that the NT is not some "all new thing, with all new ways of saying something". Rather, it is the fulfillment of the OT and it deals with the same people (Israel/Judea) and Jesus would have spoken in the same way that God always spoke to His people. No offense to anyone, but today, too many people read the NT by itself nd they read it as if it should be read like it was written by, for and about 21st century Gentiles and that is exactly the opposite of how it should be read!
Something to think about, anyway.