That is, careful evolution of internal APIs is not given much weight, so modularity - in the sense of containing change - suffers.
I don't think monorepos must necessarily go this way, but expressing dependencies in terms of build targets rather than versioned artifacts has a strong gravitational effect. Change flows through the codebase quickly. That has upsides - killing off legacy dependencies more quickly - and downsides, wanting to delete code that isn't pulling its weight because of the labour it is inducing.
[I currently work at Google but I've only been here a few weeks. I certainly don't speak for the company.]