The main reason I'm curious is because they write

'We solve this problem by allowing users to create instances of the class/interface without invoking anything on it. For classes this is done via FormatterServices.GetUninitializedObject and for interfaces this is done via Castle Dynamic Proxy. This lets us "reference" (hence the name Ref) methods on the objects without actually instantiating them with side effects. Method calls should never be made on these objects (and most wouldn't work anyways).'

Which sounds like a lot of heavy lifting. It seems something like

    public class WorkflowBuilder<T> where T : class
    {
        // Somehow workflow gets the real instance
        private T Instance;
    
        public async Task<TResult> ExecuteActivityAsync<TResult>(Func<T, Task<TResult>> func) => await func(Instance);
    
        public async Task<TResult> ExecuteActivityAsync<TResult>(Func<Task<TResult>> func) => await func();
    
        public async Task ExecuteActivityAsync(Func<Task> func) => await func();
    
        public async Task ExecuteActivityAsync(Func<T, Task> func) => await func(Instance);
    }
    
    public record Purchase(string ItemID, string UserID);
    
    public class PurchaseActivities
    {
        public static WorkflowBuilder<PurchaseActivities> OneClickBuyWorkflow => new WorkflowBuilder<PurchaseActivities>();
    
        public async Task DoPurchaseAsync(Purchase purchase)
        {
            await OneClickBuyWorkflow.ExecuteActivityAsync(e => e.DoPurchaseAsync(purchase));
        }
    
        public static async Task DoPurchaseAsyncStatic(Purchase purchase)
        {
            await OneClickBuyWorkflow.ExecuteActivityAsync(() => DoPurchaseAsyncStatic(purchase));
        }
    }

Would pretty much achieve the same thing.