This is generally solved with a 1 week onboarding processing into company culture/dev tools. Normally done by assigning an experienced engineer a week of priority time with the newcomer with the focus on showing company tools and techniques. This really shouldn't be an issue if managed even slightly.
LINQ or VS.net compiler features are not difficult to grasp.
So if your 'huge difference' translates into a week of onboarding time, then sure, you can call it a 'huge difference'.
Regardless of how good a developer is, there is simply no way that a single week will get a developer proficient in a different language to write sufficient code. Yes, any sane dev team will dedicate senior developer time towards getting a new member up to speed with the code base, but any transitioning developer working on a non-trivial project will not just pick up everything within a week. It might work like that in a beginning startup with a small code base, but it just doesn't work like that in your typical business., to truly understand how they work within .NET, and to conform to the current standard of how code is written within the company.
That doesn't even begin to scratch the surface of how long the transitioning developer has been an employee. It's very likely that this developer is brand new to the team, and even new lead developers or team managers take longer than a week to be proficient at a company.
Yes, LINQ, lambda expressions and many of the other constructs within C# are easy to understand. What takes time is learning where to use these things.
A transitioning developer needs a week of time with a senior dev and the code-base at minimum. They need at least a month to write code to the same speed and standard of another team member, and that is if they really try and adapt to their new tools and environment.
LINQ or VS.net compiler features are not difficult to grasp.
So if your 'huge difference' translates into a week of onboarding time, then sure, you can call it a 'huge difference'.