What I understood from the mentioned section of specification, it is not mandatory in case of combined attach procedure, attach accept wouldn't contain TMSI. There are two cases as following:
1. Attach accept can have either IMSI or TMSI
2. Attach accept neither have IMSI nor TMSI.
If core network (MME) can't allocate GUTI then in that case MME may send IMSI in attach accept. However it is not mentioned in the specification exactly when MME would not be able to allocate GUTI for the received attach request.
I am expecting that someone will share more information in this topic.