Let's pretend we are taking it down in dictation. How would you notate that opening? Some kind of compound duple time, right? Either 6/8 or 6/4. Here are a couple of possibilities:
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That doesn't look or sound like 2/2 to me, but maybe I'm missing something as he is relating it to the following woodwind theme. But I could certainly hear the opening in 3/2 instead of 6/4, despite the indicated phrase groupings. Actually, I think a lot of performers, myself included, try out different metric groupings when we are learning a piece. It gives you different perspectives and can add to your interpretive understanding. It seems as if what Sibelius is up to with his revised metric notation is to make the character of the opening more ambiguous, more tentative, to create a narrative in which the ambiguity of the opening leads to a later unfolding. Kallio mentions that the horns seem to contradict the meter when they come in.
The paper by Kallio goes into a lot more detail, so please seek it out if you are interested. In the meantime, enjoy the whole symphony. The last movement is a particularly successful symphonic finale in my view!
This is Leonard Bernstein and the Vienna Philharmonic. Notice that he beats all the rests of the opening measure!