The if-clause activates when the update would have reduced excess_blob_gas below 0. When pricing blobs, excess_blob_gas is the log-domain encoding of the blob base fee, where 0 corresponds to MIN_BASE_FEE_PER_BLOB_GAS (currently 1 wei). You could actually still consider moving the if-clause as you suggest. The reason for doing so would however be more obscure (yet theoretically sound): to allow a threshold just above MIN_BASE_FEE_PER_BLOB_GAS to influence excess_blob_gas, in the case where a reduction by TARGET_BLOB_GAS_PER_BLOCK would have otherwise reduced excess_blob_gas below 0.