I would be highly skeptical about Telegram as well. If I would need to select either Whatsapp or Telegram, Whatsapp would be really easy choice for me, considering the background of Durov. For some reason, Telegram is extremely popular in Russia and still has managed to avoid goverment bans.
There are various reasons in my opinion. People, who were laid off from Nokia, were highly educated and many of them were well-respected in their fields so it is quite easy to lure some new companies to Oulu (Mediatek, Nordic Semiconductor etc.) In addition, people were also used to make business internationally so it was lot easier to start new companies with that kind of experience.
If you are working in IT other technology sector, you don't have to speak Finnish, but it is definetely beneficial to have some skills in local language. In normal life there might be situations where Finnish is necessary even though most Finns speak English quite well, at least younger generations.
Current Oulu resident here, working in the IT sector as well. Less than ten years ago, atmosphere was quite pessimistic when Nokia was laying off lot of people, which also heavily affected also multiple sub-contractor companies.
However, everything has gone lot better than anyone expected and currently there are more IT sector employees in Oulu than in the golden ages of Nokia.