It depends on how the YC pro rata rights are documented. It's entirely possible that their pro rata right will be completely independent of any later pro rata right for major investors (i.e., the major Series A investors might get their pro rata rights in addition to the continuing YC pro rata).
That blog post is helpful but not the best source of info. For one, it confuses preemptive rights (right to buy a % of future financing) with first refusal rights (right to buy shares from other current stockholders who try to sell). And second, it's rather one-sided. Companies understandably want to limit these rights to only big investors for a number of reasons but especially because (1) it really can be expensive/time consuming to continually contact or chase down signatures from an investor base that eventually might include dozens of people/entities, (2) it can make it really hard to convince new investors that the investment will be worthwhile when there are pro rata rights to buy up a huge chunk of the round and (3) there's a major signalling problem when the prior angels have these rights but choose not to use them (the author mentions that he always demands these rights but doesn't always use them, which can scare off other investors and, what's worse, many angels will decline for innocuous reasons such as a seed-stage only investor who never does follow-ons or a smaller angel who is priced out by a high valuation).
That blog post is helpful but not the best source of info. For one, it confuses preemptive rights (right to buy a % of future financing) with first refusal rights (right to buy shares from other current stockholders who try to sell). And second, it's rather one-sided. Companies understandably want to limit these rights to only big investors for a number of reasons but especially because (1) it really can be expensive/time consuming to continually contact or chase down signatures from an investor base that eventually might include dozens of people/entities, (2) it can make it really hard to convince new investors that the investment will be worthwhile when there are pro rata rights to buy up a huge chunk of the round and (3) there's a major signalling problem when the prior angels have these rights but choose not to use them (the author mentions that he always demands these rights but doesn't always use them, which can scare off other investors and, what's worse, many angels will decline for innocuous reasons such as a seed-stage only investor who never does follow-ons or a smaller angel who is priced out by a high valuation).