That's just a speculative theory and an unreasonable one at that.
The more reasonable and logical explanation is that both existed in a Cushitic HOA population and the Dir forefather was simply a T carrier by mere chance and later on, it blew up in frequency in areas where his tribe settled due to his/their reproductive success.
Also, the Dir Y TMRCA is under 1,500 years. Somalis lived in Northern Somalia prior to this, most definitely. The Habar Awal E1b1b subclade and South Somali E1b1b sub don't share ancestry up to 3,100 years before present.
The Bronze Age in Egypt begins about 1640 BC, or 3600 years ago.
http://www.softschools.com/timelines/the_bronze_age_timeline/145/
From the link above.
"Nowadays, T1a subclades dating from the Neolithic found in East Africa include Y16247 (downstream of CTS2214) and Y16897. Other subclades dating from the Bronze Age (see below) are present as well, such as Y15711 and Y21004, both downstream of CTS2214."
The Neolithic corresponds to a settlement in the Red Sea Hills at 6-8 KYA, and the Bronze age corresponds to a migration south at about 3 KYA.
Your explanation that T1a and V32 were initially contemporaneous does not allow for the Dir being the oldest Somali clan or for the Northern clans to have formed in the South. It is entirely possible there were two E1b1b subclades in the South, especially since the Reewin claim a different genealogy from the Maxaa speakers. The better explanation is that a different version of V32 came from the South, joining the Dir, already in the North.