Nitrogen Fixers

So I always thought the addition of nitrogen fixers automatically adds nitrogen to the soil. But then I watched a video recently that said it’s actually only when the plants are damaged or die do they really release nitrogen. So a nitrogen fixing tree will not benefit a permaculture forest per se. It’s only by leaf, decomposition and pruning is nitrogen released. So it’s better to plant perennials and annuals that release nitrogen at the end of its life/season cycle.

I googled it and it appears to be correct. Yet everyone still seems to plant nitrogen fixing trees. So they must work, right? Because I’d rather save that space for another tree that I can benefit from if it’s better to just plant nitrogen fixing perennials and annuals in the long run anyway.