It's complicated. Reddit was founded and funded by YC in 2005, then acquired by Conde Nast in 2006, then spun off again in 2012. Since then it has attracted ~8 more rounds of funding. Advance Publications (parent of Conde Nast) currently only owns 30% of the company, Tencent 11%, Fidelity 9.5%.

Notably, Sam Altman also invested in Reddit in 2014 when he was president of YC, and now his "investment properties " own 8.7%. What the composition of that investment vehicle is, and how YC/Paul Graham are involved, I can't say.