How? By what mechanism does Hu have of undoing Xi's progress, and in what area? The big criticism of CYL in the past was corruption and inaction, that is not going to happen with Xi on top. Hu Chunhua has proven to be a good executor of Xi policy.
I shall preface that I am not the biggest fan of Jiang Zemin, but Hu Jintao and his CYL faction stagnated progress Deng and Jiang set in motion just to cater to the urban poor (not saying poor people aren't important, but at the end of the day you need to bake a bigger cake to feed everyone before worrying about dividing the cake evenly amongst the population). You can say whatever you want in hindsight, but the PRC could've gotten a lot further along during the 2008 financial crisis and had the US+EU by the balls if it weren't hindered by Hu's economic policies. The country stagnated on the innovation department, and we've been stuck with the stereotype of "copying the West and making cheap shit". Add severe corruption into the mix and China was trending downwards before the 18th Party Congress, you know its bad when Party leadership as well as its elders decided it was untenable for the CYL to continue to lead the country.
Hu Chunhua being a member of the CYL has:
- A proverbial barrel pointed to his head - Xi didn't have enough sway not to include Hu in the Politburo when the 19th Party Congress convened, he's kept Hu on the Politburo but he treads a dangerous line. There's a reason Hu spats more of the Party line and mention's Xi in practically every one of his speeches than any of Xi's allies does.
- Insignificant areas of management in his portfolio as Vice Premier... He's not in the Central Committee Leading Groups on Economic and Financial Affairs, Foreign Affairs, Hong Kong and Macau, Taiwan etc (or at least the ones that matter, à la nobody in the US gives a crap about who the Secretary of the Interior is, or who runs the Department of Agriculture). He is essentially paraded around during ceremonial events, especially when Xi doesn't want to personally meet with foreign dignitaries or even send people from his inner circle to represent him, and as such sends someone who obviously has no sway on policy making like Hu Chunhua instead. Therefore I wouldn't say he has any policy to execute in the first place.
Even if Hu Chunhua ultimately becomes Premier, he will not be in Xi's inner circle and isn't someone Xi trusts.