Looking to a standard source of definitions, the Systems Biology Ontology definition of inhibition is "Negative modulation of the execution of a process."
As such, it doesn't really make sense to talk about inhibiting a gene per se. What people are actually doing when they say that is using a shorthand to describe inhibiting either the expression of the gene or the activity of its product.
siRNA fits this quite well, as there you are talking about inhibiting gene expression. Knockout, on the other hand, means the gene isn't there to have its expression inhibited.
I would thus recommend not referring to knockout as inhibition. A better phrasing of your sentence above might be: "Knockout of the gene showed effect X, consistent with the prior result in which siRNA inhibition of the gene also produced effect X."