They invested before it was released, what are you talking about?
Tencent paid for it to be made
Edit: upon research you are talking about when Tencent became the owner. Not their investments to have the game made.
Not mentioned in a Washington Post article but
“ Over the summer of 2008, Merrill and Beck’s work seeking funds proved fruitful and the two landed $7 million more in funding from Benchmark and FirstMark Capital venture capital firms.”
/+ 10 mill from Tencent and prior to launch Tencent gave another 10 mill to buy out those investors
I can’t find anything suggesting the game was released prior to 2008
Riot released some video at sometime talking about how they had to cram to make the game even playable prior to the sales pitch so I really doubt it was made before that