The internet itself went through a similar growth pattern without astroturf. The original users were all researchers, which served as a strong implicit filter, and then the new users were students who had to be taught Netiquette every September, and eventually the floodgates opened to the public and the academics lost the ability to steer the culture in what was called The Eternal September (1993).

The same "initial implicit filter followed by gradual but inevitable reversion to the mean" dynamic explains your observations of early reddit without implying fraud, although it certainly doesn't imply the absence of fraud either. That said, "fraud" is probably a strong word for reddit astroturf in this present day and age where we have a (comparatively) planet-sized Dead Internet built on geological quantities of ads and slop.