If want to know if something existed in a community, check if there is a local name or word for it.
Kikuyu have a name for slaves but none for gays.
Even none for fish!

The e-book called “The Discovery of Lake Rudolf and Lake Stefanie” by Ludgwig von Hohnel which describes Count Samuel Teleki’s travel to Lake Turkana between 1887 and 1888.

On Page 284, it says “Abdullah had bought 200 Slaves from Kikuyuland and Kambaland on his way from the coast to Lake Baringo”.

On Page 302 there is an account of the Count Teleki’s caravan being offered slaves for sale in Kikuyuland including a young woman with a baby at her breast and with a three year old little boy at her hand all of who were then bought by Jumbe Kimameta for twenty rings of iron wire and twenty strings of Masai beads all of which were approimately equal to the value of 1 Dollar.

Here being clear evidence of slave dealing in Kikuyuland.

The natives (wakikuyu) brought food ,weapons, and ornaments in great qantities for sale. They also offered us slaves , chiefly girls from Ukambani, with a few Masai maidens. They were ready to accept almost anything in payment, but they preferred deep red Masai beads and thick brass wire. Some of our people,
who had been ailing ever since we left Taveta, ran away here in the night, probably tempted by the fertility of the land , but their fate was pretty sure to be slavery , and the natives always consider such fugitives their property. A good many caravan
men are caught in this way, but they always hope to evade their new masters on some favourable opportunity.

But we've heard how the southern Kikuyu sold Maasai refugees

Precisely!
Kikuyus have a name for slaves 'Ngombo' which means slavery existed

which book are u referring to with page 340? I'm looking for that passage but cant seem to find it

A little further tramp uphill through the narrow belt of primæval forest which forms a natural frontier encircling the whole of Kikuyuland, and we found ourselves on its inner edge, looking down upon a
charming landscape, with nothing to recall the dense woods with which it had once been covered but here and there a group of trees or a few stumps some three feet high . From the picturesque little groves still left rose columns of smoke, betraying the presence of native settlements, whilst all around them as far as the eye could reach stretched well-cultivated, undulating pasture-lands, which were a revelation to us, explaining the ease with which the Wakikuyu can supply the needs of the largest caravans.

In the ravines and valleys flow insignificant streams, and the country is almost bare of trees , but very well cultivated , the more humid valleys with sugar-cane, more rarely with bananas or colocasia ; the hill- slopes with potatoes, beans, gums, millet, tobacco , and so on.

During our march here (within wakikuyu country) we had passed through districts so carefully and systematically cultivated that we might have been in Europe.

In the light grey volcanic soil of Kikuyuland grow nearly all the cereals native to East Africa, and it is , in fact, the granary of a very extended district. Several kinds of bananas are grown as well as beans, sugar- cane, maize, potatoes, yams,
eleusine, dhurra, millet (Panicum italicum , L .), mawale ( Pennisetum spicatum ), gourds, colocasia, and tobacco . Of course all these are not equally distributed, millet, beans, and potatoes
being most plentiful in the south , whilst bananas abound in the north and millet is entirely absent.
The occurrence of millet in Kikuyuland is of peculiar
significance , as it has not so far been met with elsewhere in
Africa.

Bananas are seldom allowed to become ripe, and we could rarely get them . They are picked when still green and either cooked for food or dried to make flour. Dhurra, eleusine, and yams are also used for flour. Sugar- cane thrives admirably here, but it does not grow to the great height it attains in
tropical lowlands. The natives chew it and also sometimes make it into an intoxicating beverage.

The Wakikuyu are not only zealous agriculturalists, they also keep bees and breed cattle, sheep, poultry and goats - occassionary castrating the rams - which they are willing to sell, though it is difficult to get them to part with their cattle.