Greenpeace's analysis is demonstrably wrong, and the comments made by Lauri Myllyvirta on Twitter suggests he should learn some basic facts before rating analysis that would get at best a C if submitted as a GCSE assignment. Unfortunately journalists who should no better reported his analysis with no outside comment. This happens too often with unrigorous reports by NGOs.
To figure out what wind is replacing all you know to look at is the marginal fuel. Any electricity come from a wind farm will replace whatever is on the margin.
In Britain it appears that wind farms currently displace gas 1-1. That was the conclusion that Chris Goodall came to after analysing recent output data (and written up in the Guardian alongside Mark Lynas). After looking at the numbers myself the arguments seem robust, though peer reviewed research has yet to be done, as far as I know. The marginal fuel has overwhelmingly been gas recently, so wind really just displaces gas.
In the US things are more complex, because there are an array of regional grids. The paper below (possibly paywalled) provided estimates of the marginal fuel mixes in the key regions (see their table 1).
Some wind heavy places almost have gas exclusively as the marginal fuel. In Texas it is 84% gas. Others such as the midwest are more likely to have coal as the marginal fuel. But it is clear that the marginal fuel is more likely to be gas than coal.
This shows that Greenpeace's naive assumption that wind displaces coal 1-1 is not based on reality.