it is really hard to compare vanilla and c3c in this case.. specially with celts since they have militaristic trait in vanilla/ptw which is bad for poprushing (no 40 shield barracks), and agricultural is huge advantage for growth for the entire timeframe up till communism (four 4-turn settler factories at the starting area possible in c3c if the shields was available).
Even the difference in barbarian behavior mattered.. since in vanilla bargs will go straight for your units and cities, and in ptw and c3c (if AIPatrol=0) they will roam and only go for cities/units if they are on the nw/se axis... the early barb attack set us back a few turns.. 4 turn vs 10 turn forest chops had an inpact too, since 2 of the 4 foodsources at start was game-forests
On the other side Team Offa did one bad early move imo.. they should have buildt ToA instead of lighthouse.. free temples in every city far outweighs the inability to build universities.... this means 1 gpt less maintainance for every town, plus 30 less shields spent per town... and as it turned out they never needed education either...
In other words.. its hard to compare the c3c and classic games in this sgotm

(and those jason best dates should imo have been reversed...)
in addition I see the tech costs were 25% higher in the c3c games than in vanilla/ptw saves
Oh.. and there are higher than 3000cpt rates in HoF 100k games.. eg. my game there passed 3000cpt in 1000AD and ended up with 3112cpt in 1090AD (I stopped rushing in 1000AD, everything after that was just culture doublings).. I havent checked the numbers for the other fast 100k games, but assume they are similar