Sorry no, ironically Outliers never once mentions "deliberate" ( I checked the index) but refers to the paper by K. Anders Ericsson et al., The role of deliberate practice in the acquisition of expert performance. http://projects.ict.usc.edu/itw/gel/EricssonDeliberatePracti...

Ericsson wrote a book to correct the record. The book's lede is essentially "Gladwell bastardized my research to fit a narrative that would sell books."

https://www.amazon.com/Peak-Secrets-New-Science-Expertise-eb...

It's a good read.

Another way to think about it, is practicing at the edge of your skill / knowledge. If you can already do x, then you are at a good position to learn to do x + i (where i is an incremental improvement over x), by practicing x + i until you can do x + i. Rinse, repeat.