They’re both copyleft so no chance of a rug pull.

That's not accurate. It also takes an absence of a cla (Contributor License Agreement) transfering ownership of patches and a diverse set of major contributors to develop that protection.
GPL protects against outside entities taking over a project via a fork, owners are always free to change the license of what they made.

I didn't see a cla on either libreoffice online nor onlyoffice, but you would have to contribute some actual changes to see you don't need to agree to anything and they will accept your contributions without rewriting them later.

In comparison for example audacity makes you transfer rights over code contributions to them. That means they could make audacity closed source at any time and any version from that point would be proprietary. Would they not force contributors to sign that cla, and instead go with a copyleft contribution license, then with going closed source they would violate the licenses under which they use all these contributions.

Basically distributed ownership prevents rug pulls, since ownership beats license restrictions. So you have to check that a project has spread out ownership (independend major contributions) connected by copyleft licenses (standard unless overridden by a (non copyleft) cla)