The criticism is completely unjustified. Smalltalk has been used by numerous enterprises over the past three decades to write very large and complex software. I absolutely guarantee that these are not one-man projects.
Smalltalk works exactly the same way as in other languages like Java, Python, and C++. The members of a programming team work on different parts of the software on their own, and at some point, their work is integrated. The only difference is that a Smalltalk developer is working with a private copy of the image.
Different Smalltalk products have different ways of approaching this. VA Smalltalk, for example, uses the highly-touted ENVY source control system. Pharo uses Monticello or GitHub. Yes, GitHub! Pharo has a Git-integration tool called Iceberg.