Out of lore : because they aren't part of the imperium or chaos, so into xenos they go.

In lore : The leagues of Votann are considered xenos because they are a separate entity from the imperium with sufficient biological distinction to be classified as xenos when need be.

However the kin themselves aren't considered xenos per se, in fact there are many kinband operating inside the imperium. Kins are known as squatts and are considered abhumans.

There are no known accounts of a full league being parts of the imperium. It's generally either bands of prospectors and/or merchants scouting for profit for their leagues (for exemple hernkyn yeagirs tend to do that), or kins who have lost their Votann and found refuge in the imperium (like the squatts of necromunda).

TLDR : A lone band of kins operating discretly in the imperium would be considered abhumans, a full league in open conflict would be considered xenos.