That's not true at all. Donations are the biggest source of income for The Document Foundation, the non-profit behind LibreOffice, and have been used to pay developers who've joined the team in recent years:
* Michael Weghorn (accessibility)
* Jonathan Clark (language and scripting support)
* Neil Roberts (scripting improvements)
* Dan Williams (macOS support and general UI improvements)
How much do they pay them?
They don't disclose individual salaries but you can check their annual reports [1]. About 1M € among all the developers.
[1] https://www.documentfoundation.org/financials-and-reports/