The messaging from Intel is that DX11 and lesser APIs are going to suffer in performance initially, they have to work on individual game optimizations for games using these APIs, and it will be a continuing work in progress, they have a "most popular" game sheet they are working off.
For DX12/Vulkan games, it's not the case, it just works better due to the less complexity of the API. DX12 is lighter and most of the optimizations are already done developer side, instead of driver side in DX11 and backward. That's the reason.
So it's true, and Intel is hedging that fact right now and being very open about those facts.
Consider AMD and NVIDIA have had decades of growth alongside those DX9/11 games to inject optimizations in their drivers over time, and Intel has not. For DX11 and lesser APIs, it's all about the specific optimizations for the games being done in the driver.