Not sure if there are many gamers here but, this stake is analogous to when Tencent invested in ATVI in 2013. Tencent derives nearly half of its revenues from gaming(they own League of Legends, the largest/most profitable game in the world). Gaming is huge in China, especially among its youths, and Tencent understands the gaming landscape there well. They identified many of the most popular games in China, such as Warcraft, World of Warcraft, Starcraft, Hearthstone, Diablo - all owned by ATVI. So even though the stock at the time was stagnant for 4 years from 2009 to 2013, Tencent invested because they bet on the trends that they saw in China. Since then ATVI has gone from $15 to $50.
So even though TSLA has been similarly stagnant for the past 4 years, perhaps Tencent now sees some trends in vehicle sales that made them invest.