You control who exactly sees your online/last seen, regardless of the app friends are using. If you change it to "Nobody", nobody can. This is in contrast to other apps, which have less privacy settings and sometimes have settings that can be bypassed (e.g. Whatsapp). Nothing like that works on Telegram.
Remember the exception: If you send somebody a message or send a message to a common group with them, they will briefly see you 'online'. On iOS this is also extended to when you're 'typing'. That is because otherwise it would feel rather stupid to be receiving replies from a person 'last seen recently'.
So in other words, a user's online/last seen status in Telegram is always visible exactly to those that the user specifies in their Privacy Settings, regardless of which apps are used to view this information.