Read your own source:

>This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations.

Your source does explain that it gets complicated when there are, for example, two different kinds of mutations in the two copies of the gene. That obviously goes beyond the simple categorization of recessive vs. dominant.

Other sources make the definition even clearer, like this:

>sickle cell anemia, which is defined as homozygosity for the sickle hemoglobin (HbS) gene (i.e., for a missense mutation [Glu6Val, rs334] in the β-globin gene [HBB])