For sure, a year at a minimum, and probably more like two. *If* they take the time to go through that learning curve, then yes, nobody, not even Walmsley, is saying they would probably not dominate. But that said, the thing people here who haven't followed the ultra scene just do not seem to get is you can't simply drop someone from an elite track or road background into a trail or mountain ultra of 50 to 100 miles on a single training block with no prior ultra experience and expect them to come even close to hanging with a guy like Walmsley.
Heck, Walmsley himself is the poster boy for this, and he is ultimately probably something like a 2:11 marathon guy if he were train specifically for it. (His 1:04 half equates to 2:13 to 2:14 on several equivalency calculators, and that was with only a single 8-week training block after no track/road efforts for years.) Somebody will correct me if I'm wrong, but if I recall, it took a couple of years of training specifically for ultras and racing them before he figured things out at the shorter ultra distances and started breaking so many course records up to 50 miles. As Sage Canaday said a few posts above, in one of Jim's first major ultras, the 2014 Speedgoat 50K, he just got killed by all the uphills/downhills.
And even after he seemingly mastered the shorter (50K up to 100K) ultras and began his record-breaking sprees for those in maybe 2015 or 2016, it took him longer yet to finally figure out the 100-mile distance at Western States. Even though I am a fan of Walmsley, I consider him to have gotten "lucky" in his first attempt in 2016 when he was under record pace for 93 miles and managed to hold everything together long enough he almost won before going off-course and flaming out.
I say that because the next year at Western States in 2017, he didn't accurately gauge how the heat would affect him at the faster pace he attempted (in 2016 it had been slightly cooler, though still hot, but did not seemingly affect him too much, so this mistake was understandable), nor had he yet figured out proper nutrition strategy for races longer than 50 miles or 100K. The heat combined with the faster pace and suboptimal nutrition caused him to start upchucking repeatedly after two-thirds the way into the race forcing him to drop out.
Finally in 2018 after working with his nutrition sponsor (Clif), he got his nutrition formulations and intake strategy dialed in to prevent stomach upset at the longer distance. He also showed more prudence in moderating his pace the first half of Western States so he was able to run strongly throughout and break the course record, even in weather hotter than 2016 or 2017. That kind of knowledge is hard-won and doesn't come quickly.
Even then, he proved unable to follow up after Western States with any success at UTMB. My guess would be probably because Western States went so well, he did not realize/feel he needed to take more recovery time before gearing back up again for another 100-mile like UTMB 10 weeks later, saying his legs felt "flat" from the very beginning of the race, and eventually dropping. He seems to be an animal when it comes to training mileage, but there are limits and it looks like it's taking him a while to figure out where those are for him.
By contrast, the much more experienced Francois D'Haene, the prior-year winner at UTMB, and 2nd at Western States behind Walmsley, didn't even consider attempting UTMB in 2018.
It takes a while.