Once I left the cap off my bottle of Sweet's copper solvent... forgot about it for a few days... the ammonia practically all escaped!
Then it was useless...
Ammonia evaporates, likely way too fast to do any harm to your brass if you just put a tad of Brass in the tumble media.
The horse urine thing that happened to the Brits is kinda funny... and I wouldn't necessarily dispute that, but the ammonia was present continually, and there were likely other factors which contributed to the brass cases failing.
All that said, erring on the side of caution is never a bad thing, especially when there are other products that'll do the same job with no ammonia.
Dan