Passholder, here
This is true but passholders already have access to the other two parks so it makes sense we wouldn't be forced to buy combo tickets for access we've already paid for.
Given they announced this when tickets first went on sale, essentially saying "passholders, hold on for more details on buying tickets" and then released tickets a couple weeks later, it seems this was a planned rollout from the start rather than a reaction to soft sales - giving a small head-start to people rushing to buy the 3-day before opening it up to passholders (who Universal still actively markets to in a way Disney doesn't, anymore) and then, much further down the line, opening single day purchases to the general public for what's left.
It makes sense because if they were going to sell out early days, let that be for the people willing to buy the 3-pack. Passholders aren't as likely to go crazy buying a week for Epic because many are local, not going on an actual vacation to visit and the rest are price-sensitive which is why they opted for an AP to begin with. Waiting to open up the anything-goes single tickets at the end to fill in the capacity that remains seems like a smart strategy.
That's not to say ticket sales aren't soft - just that their current strategy seems to be planned and not any reaction to ticket sales, good or bad.