I totally get Arkansas fans (and others) being unwilling to concede that Cal actually develops his players (it's hard to acknowledge the strengths of a rival... or, in this instance, a would-be rival). We can disagree on whether or not Cal develops his players (and I do disagree), but there is no debate as to the benefit of a player like Monk choosing Kentucky over Arkansas.
While it might be accurate to conclude that most of the 5-star players Cal recruits would still be drafted by the NBA regardless of where they decided to play in college, that is certainly not the case for all of them. To cite just one example, Malik Newman, a very similar prospect to Monk who was actually ranked higher in his class than Monk was, clearly made a terrible decision when he chose to stay home and play for Mississippi State rather than come play for Cal at UK. He fell completely off the national radar and was not able to showcase his skills. Perhaps he was just not as good as advertised (that happens sometimes), but even for the guys who live up to the hype, it's generally a poor decision for their personal brand and development as a player and future pro to go somewhere out of the spotlight. Ben Simmons was still drafted #1, but his year at LSU was mostly a disaster and he went from the clear cut #1 draft pick to a guy with a lot of questions dogging him and at least some thought that the Sixers might select Brandon Ingram over him). Markelle Fultz is having a terrific freshman year at Washington and will still be drafted high, but most of America has no idea who he is because they never see Washington play or hear anything about them and he will likely miss the NCAA Tournament.
As for player development, I think the Hog fans (and others) are not fully understanding what "development" encompasses. The list of already great players Cal has helped develop into more well-rounded players is long (just watch Monk's first 2 games at UK and compare it to what we're seeing now... he's a completely different player now who plays under control at all times, knows the difference between a good shot and a bad shot now, and actually plays defense now... still working on rebounding, but that will come). But playing at UK offers so much more crucial development than just what happens on the court. The spotlight is bright here both from a local and national perspective and the kids are learning from day one how to be a professional - how hard you have to work, how to deal with the media and intense public scrutiny and fame, how to play with other star players, and how to give back to your community (the servant leadership that Cal is always preaching).
Kentucky is not the only school that signs these elite one-and-done players, but it is indisputable that UK's guys generally adjust much quicker to being an NBA player and blossom into star players at a higher rate than the other schools' players. That's not a coincidence.