It's part math problem, part music logic, isn't it....
And you're right -- taken literally, the lower 2 staves look like 4/4 time.
So.....what to do with it.....
My first premise is, the rhythm of the upper staff is exactly what it looks like, and the basic 3/4 is maintained.
Which means that everything else has to fit within that.
So......looking at that first measure....
The lower 2 staves are clearly indicated as being associated with the upper staff in a very specific way. The 2 quarter-rests correspond to the 4 eighth-notes above them. Those quarter-rests are just what they look like: each has the length of a quarter note.
That leaves us having to figure out what to do with that low note and the couple of riffs in the first part of that measure.
I think we have no choice but to conclude that the indicated note values either:
-- weren't meant literally, or
-- Debussy or the editor or typesetter just made a mistake on them.
Be that as it may, whichever (doesn't matter)......
The low note plus the quintuplet occupy the first half of the first beat, i.e. a total duration of an 8th note.
The next figure plus the 16th-rest also occupy the length of an 8th note.
I think that summarizes the heavy lifting on this. The rest is just scribbling. (As Mozart says in Amadeus
In order to make this 'work,' we have to either not worry about the indicated note values, or to imagine that they're written double too long
-- i.e. the 8th-note stem on the low note should be a 16th note stem, the 32nd notes should be 64th's, etc.
There are other slight complications in the decisions, such as how to regard the quintuplet, and what to do with the second measure, but I think once we have these basics in mind, those are relatively simple and you can figure them out (as could anyone else who's advanced enough to be really delving into this).
So -- whaddaya think?
edit: I see that Dan covered this too, and basically the same, only with a lot less words.