This silly.
It opposite.
It thagicu that moved North.
Sonjo didnt move back.
Sonjo-Thagicu had lived with Dagota-Iraqw in Tanzania - and we see it with Loan words.
Loan words for Donkey, Rhino, Wilderbest, circumscion - these are animals that were new to you bantus - who managed to cross the Serengeti animal kindgom.
They Thagicu-Sonjo all copied from Datoga.
Donkey for example is Digiri (Datoga) - kalenjin is slightly different - Sigiriet.
Then as they get to Mt Kenya.
Thagicu start to borrow from Kalenjin living there (Dorobo/Gumba/umba/lumbwa)
We see Thagicu borrow Mutarakwa - cypress - from Kalenjin - as it grows in that region.
And many others word.
Then of course you borrowed from Maasai, Borana and Mijikendas too.
And Chagaa-Taitas.
Sonjos are well known in Kipsigis.
They have lived with Kipsigis-Datoga --- Kipisigis had contact with TZ Datoga - friendly terms.
Sonjoek is nick-name for 'smart or mjanja' in kipsigis - I guess they were as craft as kikuyus.
Bringing us to Meru migration story.
I think it borrowed from Kalenjin-Datooga.
It very similar of being imprisoned by red people (kipyayamugen or something)
This paper is a selective synopsis of an article that will appear in volumes 12/13 of 'Sprache und Geschichte in Afrika'. It also presents additional information which was gathered during fieldwork in the Sonjo area (northern Tanzania) in 1992 and which has led the authors to modify their previous views in places. The paper focuses on those aspects of Sonjo which bear on Engaruka, situated at the dry foot of the rift escarpment between Lake Natron and Lake Manyara, Tanzania. First, a genetic connection for the Sonjo language and a migration pattern for the Sonjo people are established. The authors show that Sonjo is a member of the so-called E50 group, that is the Bantu languages of Central Kenya, and in particular a southern subgrouping. From this they conclude that the ancestral Sonjo community moved south from the general area of the Central Kenya Highlands. Using linguistic arguments, they then show that Sonjo's most significant contacts had been with a Datooga community (southern Nilotic). The Sonjo and the Datooga are the most likely candidates for having introduced irrigation. However, the evidence suggests that neither brought the system with them. It was set up in situ, most likely by the ancestors of the Sonjo. Finally, the authors argue that the Sonjo are the best current candidates for the former Engaruka community, which was abandoned two or three hundred years ago. Bibliogr., notes
There language is close to daatooga so they are kinda southern nilotic
https://www.africabib.org/rec.php?RID=125213816