No, it's because ibuprofen can make the lithium level in the blood rise.
And it's actually true and it happened to me! I thought it was ok to take an ibuprofen once in a while eventhough I also took lithium. My lithium level was 0.6 and I was told to increase my dose with an extra pill (300mg). I stopped taking ibuprofen cause I could handle my pains. After a week I had my blood test again and it was 0.5.... It was lower than before I increased my dose!
Ibuprofen had increased my concentration in my blood with about 0.2. And I only took a pill every other day or so.....
So from my experience you should be careful with mixing nsaid and lithium.