I am sure they are using Javascript/typescript on the front end but assuming a dotnet shop is using node as a web hosting framework is a bit of an assumption. It is just as likely that they are using kestrel or even IIS with MVC plus a client side library.

If that is what their stack looks like they could convert to building using node which no one has expertise in or they could switch to python where their new architect will be most effective.

As a dotnet developer myself I personally would stick with dotnet core but there is a case than can be made for adding in python in areas where the cold start is going to be the most costly area and you don't need something as strict as C# or the .net frameworks. That same case could be made for node as well but it depends on what other kinds of factors they are looking at within their business.