This is certainly not the case. Some, but by no means all, of the core maintainers prefer the unicode glyph, but the vast majority of the code uses a spelled-out lambda.
In my current checkout of the codebase, I see the following:
~/src/racket/racket/share/pkgs$ ag lambda **/*.rkt | wc -l
1041
~/src/racket/racket/share/pkgs$ ag λ **/*.rkt | wc -l
163
In my current checkout of the codebase, I see the following:
Note also consistent usage of spelled-out lambda in the Racket style guide, https://docs.racket-lang.org/style/.