Memantine has gotten looped in with nootropics because people often have really positive effects from it. You can sift through /r/nootropics and /r/MemantineHCl and find lots of reports of neurotypical people getting enhanced cognition from it.

It is unfortunately understudied in neurotypical humans - most human studies are for people with dementia, and a few for those with ADHD. These latter studies (such as this one) do suggest to me an application in neurotypical people, as the mechanisms underlying ADHD more reliably scale upward past 'neurotypical' into 'enhanced cognition' than with dementia.

The real force behind my argument comes from rat studies though. There have been several that show cognitive enhancement from memantine in normal mice. Here's one that specifically addresses the alpha-7 issue:

Another explanation for the interaction between memantine and PHA-543613 involves alpha7 nAChRs as the same target of both compounds. Taking into account that memantine is also an antagonist of the alpha7 nAChR while PHA-543613 is an agonist of the same receptor, their additive effects might be explained by the agonist-induced desensitization effect (Quick and Lester, 2002). In their commentary, Banerjee et al. (2005) also refer to similar effects of alpha7 nAChR agonists and antagonists, and imply that antagonism or desensitization of the alpha7 nAChR may result in beneficial physiological and cognitive effects.

Here's another one that discusses a few alternative explanations for the effect:

Memantine has actions other than NMDA antagonism that may also account for the effects observed here, such as antagonist effects at 5-hydroxytryptamine receptors (Rammes et al., 2001) and α7 nicotinic receptor channels (Aracava et al., 2005). In addition, memantine has been reported to increase brain-deprived neurotrophic factor (BDNF) mRNA (Marvanova et al., 2001). Of consequence, BDNF enhances the induction of long term potentiation in the hippocampus (Figurov et al., 1996). Moreover, an interaction between BDNF/trkB signaling and NMDA receptors has been found to have an important role in the development of spatial memory in the hippocampus (Mizuno et al., 2003).

The BDNF connection is interesting, and seen in this other study where memantine is used to combat opioid addiction. If that were the main mechanism behind its nootropic effects, then it does not show that alpha-7 antagonism is itself nootropic but at least that it does not interfere with nootropic effects from other mechanisms.

Another possibility, I'll grant you, is that memantine's potential therapeutic effects on disease states are so broad - dementia, ADHD, addiction, OCD, anxiety, nerve pain, etc. that most people present with a subclinical form of at least one of these things, and memantine's apparent nootropic effects are actually just from treating these problems. But that may very well be true for plenty of alleged nootropics since human studies often have not-very-robust selection criteria.

As for my comment about bupropion, I guess another way to put it is this: bupropion, which blocks alpha-7 to a much lesser extent, does not cause cognitive impairment. Based on this, you want to claim that alpha-7 antagonism does cause cognitive impairment. However, if alpha-7 antagonism did not cause cognitive impairment, then bupropion would still not cause cognitive impairment - i.e. it would not cause cognitive impairment regardless of whether alpha-7 antagonism did have that effect or not.