Please cite LibrePlanet first, for two reasons:
It used only free software in 2020 already.
It is a free software event, whereas FOSDEM is not.
What about we rephrase it as follows?
"We would like to highlight that an online conference hosted 30,000 participants using only free software (Matrix and Jitsi only) https://matrix.org/blog/2021/02/15/how-we-hosted-fosdem-2021-on-matrix/. It shows that for large conferences also we need not depend on proprietary software." without mentioning their name.
The point is to show that even large online conferences can be handled using only free software.