In this discussion, the symmetry is tested on the letter sequence. Arabic shaddah marks a doubled pronunciation of one written consonant, but the text treats a geminated letter as the corresponding ungeminated letter for the purpose of qalb.
The bāʾ in “fa-kabbir” bears shaddah. Counting it as two separate written positions would obscure the reversible form of “rabbaka fa-kabbir.”
This is a convention of verbal analysis, not a license to omit gemination in recitation. Correct pronunciation and the letter-based test serve different purposes.