Hi Sam,
In large parts I agree; in this scenario BizX and SAP HCM are quite different from a data model perspective and the integration can be complex depending on your scenario. However, in SAP's Hybrid model the BizX suite would be your system of record for Talent unless you did reporting out of SAP HCM for Talent using, saw, BW. For Talent you largely only need employee and organizational data, skills and competencies, and salary or performance data (depending on the processes being used). So, it doesn't have to be all that complex 
.
But I feel a Hybrid approach does add value because you are taking your core HR Master Data and using it in best-of-breed Talent Management applications that consistently top all of the top analysts' review of Talent Management suites.
SAP don't consider using SuccessFactors Employee Central with SAP HCM as Hybrid model and, by your definition, rightly so. But I think having multiple versions of the Hybrid name for different types of the model doesn't really work... or does it? Interested to know your thoughts.
Best regards,
Luke