The simple answer to this question is that not all software at Xerox PARC was written in Smalltalk. In fact, Smalltalk was just one out of a few different languages and programming environments that Xerox PARC used. For example, the Bravo word processor was written in BCPL (https://en.wikipedia.org/wiki/Bravo_(software)). Mesa was also developed at Xerox PARC and was commonly used as an implementation language. Interlisp was also largely developed at Xerox PARC.