First, I think that those employers were confusing Globish or International English with either Basic English (originally proposed by C. K. Ogden) or Joachim Grzega's Basic Global English.
Ogden's Basic English had a core vocabulary of 1200 words that could be expanded to as many as 2600 words. Grzega's Basic Global English (BGE) used a core vocabulary of 750 words.
Since some of the comments mentioned "Leichte Sprache", it is important to bear in mind that "Leichte Sprache" and "Einfache Sprache" have a different goal than "Global English", and the two concepts are sometimes confused:
Leichte Sprache is the simplest of the two and is primarily aimed at people with cognitive impairments.
Einfache Sprache is a bit more complex but still avoids foreign words and complex sentences.
(See Leichte und Einfache Sprache – Versuch einer Definition and Worin besteht der Unterschied zwischen der einfachen und der leichten Sprache? for more details about the differences.)
Leichte Sprache is written according to specific rules (see e.g. Leichte Sprache - Ein Ratgeber), but I am not aware of word lists for Leichte Sprache or Einfache Sprache. (Similarly, there are no word lists for TestDaF.)
The goal of Leichte Sprache and Einfache Sprache is not to provide a simpler language for non-native speakers, unlike "Global English". So when you look for vocabulary lists for German, you are most likely to find the following two types of resources: vocabulary lists for school children in Germany and vocabulary for people who learn German as a foreign language.
- German as a foreign language (Deutsch als Fremdsprache / DaF), see e.g.
- Vocabulary for German school children:
How to speed up your language learning is a very different question. There are several places where you can ask for advice on this, e.g. Language Learning Stack Exchange.
You can also read Gabriel Wyner's article Is there a more efficient way to learn vocabulary? Wyner is a great fan of space repetition systems such as Anki. He recommends learning the 1000 most frequent words in your target language as a way to get started. You can find word frequency lists for German on Wiktionary.
Update in response to tofro's comment "I honestly don't know of any place, book or course that would aim at teaching Leichte Sprache."
Leichte Sprache is, indeed, not a language-teaching tool but for native speakers of German. There are several books for people who want to use Leichte Sprach, and they are clearly not language learning resources for German as a foreign language: