That is make believe story.
Meru-Embu-Kikuyu is close - dont argue - you just have Ogiek/Lumbwa/Umba influence of "MB" before some words.
Tharaka is Mbeere+our kalenjin Ogiek;
Huyu DP kwanza ni kama ni Ogiek kwa clan yake
Mbeere is Kamba+Embu.
Kambas is one with some Tana River Pokomo influence.
Overally Thagicu group likely meet Chagga-Taita group in Mt Kilimanjaro.
One Taita group is admixed with Mijikenda (coastal bantus who are related all the way to Mozambique & malawi)
The Kamba group met Pokomo - Mijikenda group.
You story about migration of red people enslavement in island is VERY KALENJIN.
Pundit try another theory 
Missing link between Gusii and Kikuyu
once you accept Meru is a very distinct bantu language apo ndio utaanza kuelewa actually Kimeru is older than Kikuyu or even Kamba. Some.Meru words have no equivalent in Bantu then again Meru is not 100%Bantu only 60% or so.
Meru history and language were modelled in Tana river and coastal lamu area our migration again is different from Kikuyu.even in that article Tana river is clearly pointed out.