It is not that easy. Companies do not fund developers for LibreOffice directly, but the developers of the companies are paid for working for the customers of the company. If LibreOffice is part of that work, then they will contribute their patches to the community.
If you look at the logs in the code repository and at the mail addresses of the authors you will find companies and persons which often contribute. I count about thirty employees from that for the last month (and about fifty individuals). libreoffice/core - main, development code repository
Besides the mentioned ESC, you can look at the Advisory Board. Advisory Board | The Document Foundation - The House of LibreOffice and Document Liberation Project
The Document Foundation does not employ developers for LibreOffice. TDF employs people to maintain the infrastructure and for running the foundation and strengthen the community. Sometimes the TDF makes a tender for a topic and companies can apply for it. Tenders Archives - The Document Foundation Blog
In addition, you should consider, that there are individuals who contribute not so often and in their spare time, but nevertheless have a deep knowledge in the code. And you should consider, that not only improving the code, but translations, quality assistance and marketing are essential for the success of LibreOffice too.