A Google search produced the following explanation…

”The plural of "spelling" can be either "spelling" or "spellings", depending on the context. When referring to the general concept or a single instance of spelling, the plural is "spelling." When referring to multiple different types or instances of spelling, "spellings" is the appropriate plural form according to WordHippo and Preply.”

I read Dave’s comment as referring to ‘multiple different types or instances of spelling’, and I therefore see no problem with his use of ‘spellings’.