Does Shopify use Bazel? I would have guessed not. In any case, the more natural fit for them would be Nix, since it should be good at eliminiating redundant builds, and they recently went all-in on it.
AFAIK Bazel and Nix are orthogonal and actually go well together. Nix is great for packaging and environments, but is anemic at actual build tooling (a big one: no incremental builds within a package). Bazel does very little to manage the environment or packages, but it has very granular and efficient build support.
True, but they both require a rather large complexity and learning and complexity budget. It would be difficult to introduce both into a shop at the same time.