I think Desailly, Gallas, Cole, Cech, Lampard, Drogba, Zola, Essien, Hazard, Mourinho, Abramovics money, etc, etc, etc played a huge part too. Terry has generally been part of a strong defensive unit too.
Just because Terry's played there for ages doesn't make him good, great or GOAT (I'd say he was very good) in the same way Giggs played a part in United's success but wasn't the key part.
The list of players above (off the top of my head) includes much better players (and better defenders) and players who had a huge part in changing Chelseas fortunes from also-rans for half a century to contenders.
Terry was a cog in 2-3 very strong teams, nothing more nothing less.