I'm confused. You start the article talking about a XenApp migration tool. Then go on about migrating users from VDI and how limited the VDI marketplace is.
XenApp and VDI are two difference things. Whilst I agree that VDI has a limited penetration in many large companies, XenApp is a different kettle of fish and is probably used in virtually every FTSE100 in some way or other (remote working, application delivery etc).
It's the XenApp not VDI customers that VMware are trying to poach. VMware used to be a VDI-only player but with their latest Horizon suite they want a slice of the lucrative application delivery/presentation market that Citrix has had to themselves for years.
Please don't make the mistake using XenApp and VDI interchangeably, they most certainly aren't the same.