I am not sure about this move.
Some of the core Express.js developers have started koa.js, to try out how to build a modern framework using ES6/ES7 language features. Koa.js v2 is around the corner, and now IBM has given Express.js into the hands of the Node foundation. IMO it's either a try to keep Express.js alive as it is or attempt to force a rebase and merge koa.js and it's developers into Express.js back again.
It's good and bad IMO.
bad, because it's may now harder to replace Express.js with something better
good, because it's now easier for new Node.js developers to get started
bad, because it can develop pretty quickly into something Java-like or Rails-like (some may like to work with big bloated monolithic things, me not)
good, other projects may also join the foundation