You had asked for assistance and expertise on using R/RStudio. Unfortunately, I have never used them.

maybe R implicitly converts them

Judging from your results, I'm sure you are right.

The bottom line seems to be that the task coefficients do a much better job of predicting the completion time than do the programmer coefficients.

Yes, and if you added some additional tasks into the mix - tasks which took hours or days to complete - then programmer ID would seem to make even less difference. This points out the defect in my suggested data-analysis strategy. A better approach might have been to divide each time by the average time for the task (over all programmers), optionally also taking the log of that, and then exclude the task id as an independent variable. After all, the hypothesis is that Achilles is 10x as fast as the Tortoise, not that he takes ~30 minutes less time regardless of task size.