This seems to be something a lot of people believe, but when you look at the history of it all it's really not the case.[1] I have a feeling it's because the controversy sparked a number of articles which talked about things in a more public way which made it feel like Wayland was suddenly making progress after stagnation, while in actuality development pace was chugging along as it always had been.
[1] Canonical announced Mir early 2013[2], you'll notice there is no great spike around that time in any of:
[1a] the Wayland repo https://github.com/wayland-project/wayland/graphs/code-frequ...
[1b] the weston repo: https://github.com/wayland-project/weston/graphs/code-freque...
[2] http://www.omgubuntu.co.uk/2013/03/canonical-announce-custom...
From the Qt/KDE side of things we have work on QtWayland[2] and work on Plasma/KWin[3]. The former had development increase as work on Wayland continued and follows a general upward trend, especially after Plasma started using it. For the latter, the article covers the timeline pretty clearly.
From what I can tell my point still stands, even when talking about support from integrators.
[1] https://wiki.gnome.org/Hackfests/WaylandHackfest2012
[2] https://github.com/qt/qtwayland/graphs/code-frequency the spike you see happened Jan/Feb which was prior to the Mir announcement and if you check the number of commits there was no major increase at that time
[3] http://blog.martin-graesslin.com/blog/2013/04/the-history-on...
But now that you mention it, it's true that wayland was stagnating and that when Mir arrived, everything accelerated. Sometime having a common enemy is a good motivator.