No, it's all the engine. Of course there are variables, mostly in rider position which affects wind drag, and the bike does matter, especially the tires which affect road resistance, but when all is said and done, it's the engine.

Just take a look at what breakaway riders who are essentially time trialing can do and for how long before the peloton swallows them up.

BTW- if the segment is flat, this is the one cycling condition where size helps. Bigger heavier riders have comparatively more power, in comparison to the added wind drag that their size causes.

But being slower that top riders is no reason to get discouraged. After all 31 out of 540 isn't bad at all, but the air does get thin near the top. Years ago when I was at my peak, I was probably in the top percentiles of rider speeds, and was comfortable knowing that I could keep up with top riders until they turned it up that last notch and waved goodbye. I knew that close was OK, but that's as far as I'd ever get.