‘Compound’ as opposed to ‘simple’ meter means groups of 3s rather than 2s in a bar. (And ‘complex meter’ means a mixture of both.) Just because a given 6/4 is too slow to conduct in 2 beats doesn’t make it not-compound. If the quarters are grouped as 3 pairs, that’s ‘simple’ (regardless of the tempo or the conducting pattern), and the fact that people write 6/4 a lot where technically it’s 3/2 confuses this issue. That isn’t compound meter, but 6/4 in the Saint-Saëns is.
Then when his 3 bars of 2/2 roll into one bar of 3/1 with the accelerating tempo, that is still simple meter: 3 whole notes, each of which divides in half, quarters, etc. – now conducted in a big 3 instead of 3 pairs of beats because it’s faster. This is pedantic because of course we can easily hear the rhythm of music across barlines. Saint-Saëns could well have stayed in 2/2 (in one) to the end, but 3/1 meter conveys a sort of 19th century gigantism that’s perfect for the music.