A lot more high quality boxing training. That investment seemed to be night and day for Strickland and I wonder how much it's emphasized by Izzy. Guys with good boxing fundamentals have always given him trouble.
He didn't fight all that different than he ever does, but he did seem a bit flat. That life or death energy needs to be there, like he had in that 5th against Gastelum.
I'm sure there's a lot more.
Even then though, Strickland is a very tough style matchup. His boxing defence was exceptional, completely nullified Izzy's kicks. His reads, reactions and reflexes were great. He left very few openings to be countered or even hit at all.
I could see that Strickland beating any version of Izzy.