I think what that author meant was, if you want to link it to aikido, not develop speed but train hard.
You can compare it with running: if you start with fast running, you can always switch back to walking (marathon style for instance). If you start with marathon it's much harder to step up and go for fast running (you know: 100 metre, 60 metre).
So, I think it's not speed that counts. How will you develop a firm stance if you always go fast? There is a difference between hard training and fast training.
I hope I made a clear point!hehe.