The who found the Vulnerability where at least talking to the people at chrome and firefox:
Quote from there Blogpost (https://medium.com/bugbountywriteup/zoom-zero-day-4-million-...)
"Apr 10, 2019 — Vulnerability disclosed to Chromium security team.
Apr 19, 2019 — Vulnerability disclosed to Mozilla FireFox security team."
So, not entirely unrealistic that the other Browser manufacturer also got a notice.