Your experience is 100% compatible with the linked article: the seven-figure spender is presumably running a much higher margin business and can scale narrowly profitable ads much more effectively. The natural equilibrium for a perfect ad market is for the ad spend to be exactly equal to increase in revenue: a perfectly efficient market with no profit for the advertiser. Google (and Meta et al) are so good that for many SMBs they are completely cornered at the zero-point: spend as much as you can just to stay in the same place financially.