They were convinced that once Fergie retired they had a free ride back to the top and we'd end up in mid-table obscurity. Moyes coming in and us finishing 7th while they mounted a one off title charge just cemented that viewpoint.
However, since then, despite us still not returning to our Fergie-led best, we've won more trophies in the last two seasons than they have in their last twelve. They've not know how to react to this, which is why even among the more reasonable Liverpool fans, they're coming out with nonsense about average finishing positions, number of years finishing above United, and weirdly taking the piss out of the last two trophies they've won, as well as the Europa League, which they failed to win.
They've spent the last 25 years desperate for even the smallest piece of Premier League success, but have instead had to endure United dominating proceedings, with even Blackburn and Leicester getting a slice of the pie before them. They've spent the last 25 years absolutely convincing themselves that every coming season is their year to return to the top of the pile, only for many of those seasons to produce nothing but abject mediocrity as multiple teams leave them in their dust. They consoled themselves through the 90s and early 00s with cup wins, but they dried up 10 years ago, which is why they moved onto "net-spend this" and "golden glove that."
Klopp coming in was supposed to signal yet another return to the glory years, but after bottling two cups first time around, scraping 4th the second, and looking on the verge of losing his star player before the third has even really began, they need to lash out elsewhere.
They've spent 25 years celebrating league titles in August, only to be out of the race by January. They look at us now, and see a team that maybe, just maybe, is starting to come together, under one of the most decorated managers still in the game. They've gone from 100% certainty that we were finished once Fergie retired, and 100% certainty that they were going to fill the void, to four seasons later, where we've won three major trophies to their zero and are potentially finally beginning to click, while their progress has been marginal at best.
With that in mind, it's quite easy to see why they think we're acting as if the title is wrapped up already, because it's exactly how they'd behave in our situation. It's far easier for them to project their boom-bust-cycle onto us, than face the reality that their balloon has burst before it was even inflated properly.
As you've said, United fans claiming league titles already are few and far between. Most, if not all that I've come across are tentatively optimistic about some form of title challenge this season, and even then a lot of that is born through rose-tinted expectation than reasoned appraisal of the strength of our opponents.
We've started well, and we've started better than them, but there's a very long way to go yet and there are more teams than Liverpool to be worrying about. We can leave them to their belief that we're still playing turgidly boring, negative football week in week out, and they can keep pretending that shipping three goals to relegation battlers is entertaining, and hopefully come May, we'll be in the title mix and they'll be bemoaning bad luck and refereeing decisions as an excuse for them being 6th.