Sync is always paid, and optional. A commercial license and sync are separate.

They list it pretty clearly, but most people probably don’t read it. This is a bit troublesome since it’s in flathub and very easy to install. Probably routinely violated, that’s the danger of proprietary software. https://obsidian.md/license

Commercial Use Licenses are required whenever Obsidian is being used for work for a business with two or more personnel. Sole proprietorships or other one-person organizations do not require a Commercial Use License. Work for educational purposes does not require a Commercial Use License.

Non-profits are also exempted.

They say on their website that as soon as you use it for commercial projects, you need to get a paid plan. Their own sync is only availble on a paid plan iirc, so you should be fine.