To my knowledge (and research) Seagate does not make a solid-state drive. Only a "hybrid" drive.
A hybrid combines on-chip memory (more than a simple cache) and HD platter(s).
This sounds more like an "emergency backup" should STEC encounter problems in manufacturing true SSD's for EMC's enterprise drives. However, I suppose it's possible STX has begun qualification of a previously undisclosed SSD part.
Prudent move on EMC's part. It's always good to have an alternate source for mission-critical components.