The clinical utility of this agent (and zanamavir) has never been compared with amantadine and rimantidine in a head-to-head trial AFAIK, but in practice the older drugs are much more potent.
In the rare human cases of H5N1 disease, death is fairly rapid, thus, it would be surprising if basically weak drugs had any effect at all.
If I recall correctly, Tamiflu is effective against some strains of the H5N1 virus--China, never a very reliable reporter, insists that it is effective in the Chinese strains. That may be because Chinese farmers were routinely using amantadine (again, if I remember correctly) to prevent avian flu in their flocks.
The VN strain has been shown to be more resistant to Tamiflu, although anyone who has survived has been treated with it. Some authorities say the dose should be doubled to be effective, but I have a concern that the side effects may be more severe.
Amantadine is a very potent drug with severe side effects used to treat Parkinson's. At one time I thought it would be very useful. I no longer think so.
On the other hand, in the worst case scenario, who wouldn't try everything possible for hope?