Military personnel use the same Yandex services as regular people: delivery, maps, taxi, etc.

Also Yandex is a large cloud provider, they host a lot of government-connected companies.

Also Russia has been targeting Ukrainian digital infrastructure so they make these things fair game IMO.

I don't have specific information about Yandex but don't forget how heavily involved Amazon were with things like: https://www.nytimes.com/2022/12/19/world/europe/ukraine-cybe... it wouldn't be a huge stretch to suggest that a lot of cloud hosting providers are neck deep in national security.