Hmm, I guess they could have written a lot of code lines, but its not like the code they write would be equivalent to million LOC of a coherent whole.
I'd have guessed it was better for a training set to have lots of different mostly-known-good solutions to common problems and good visibility of the solution space, rather than one linux-sized program, even with the same bug rate between the two. I'd imagine leetcode is monetizing their user generated code this way too. IDK.
I do actually have a decent GPU i bought 4 years ago in my PC, so maybe I’ll do a few simple experiments again… my preferred approach is to pick a simple but nonlinear problem e.g. binary multiplication of two large numbers, where you e.g. skew the training sample distribution or omit some number pairs and see if the resulting model can learn correct multiplication anyway.
For that particular experiment, I'd also point you towards some of the 90s literature on "genetic programming", because you could reuse most of your scaffolding for it. No GPU req'd, ought to learn it fairly quickly.