They live and breath numerical linear algebra and are comfortable reading advanced theoretical books or papers.
It's easy for them to pick up the basics needed to pass interviews and find a data science job. How would they go about adding some rigor to their understanding of ML and statistics?
and this for statistical ML: http://statweb.stanford.edu/~tibs/ElemStatLearn/
One more interesting thing I have observed in data projects failing: organizations culture around data and the gap between data science team and engineers. Say, you have 2 top notch data scientist who know enough (stats, markov chains, algorithms and so on..). But let us say an average engineer in the organization doesn't know even a bit about A/B testing or difference between building a machine learning model Vs. obtaining predictions from already built model. Then no matter how good your so called data scientist are, the end result in terms of product or solution delivery is always sub-optimal. If the engineers and data science teams can't speak a common language, the result is always disastrous. Note that the gap is specifically about understanding data analysis as a domain.
The efforts to narrow down this gap must be driven by the lead data science member or CTO. Something like 'data bootcamp' mandatory for every new joinee can help. I had read about Facebook having such a bootcamp mandatory.
In summary, it is important to iterate quickly and to validate your results. Using complex models, like gradient boosted decision trees, can often iterate much more quickly than simple models because you don't have to do extensive data preparation. Many analysts are stuck in the mode of using linear or logistic regression for every problem, when there are better tools out there.
Also you misspelled "breathe". As in "live and breathe".