But there should be a way to force it work on a legacy gpu
Based on what? As I said, that forum post is 10 years old. It's quite possible they removed or stopped maintaining that old code, because it's not worth their time anymore.
Actually, if you scroll all the way to the bottom of their requirements, you'll see that it was removed:
The end of support for ATI/AMD 4xxx series and below was announced in March 2011, and scheduled to end in September 2011. The last of the AMD Fahcore_11 (GPU2) work units actually lasted until June 2012, more than a year after the original announcement.