You're right, but this has always been the trade off with tools like this. You put some trust in the tool's authors and gain some insight in return. Remember the services that tested for Heartbleed (e.g. https://filippo.io/Heartbleed/)? Fairly similar trade-off, but still these tools were widely used.

If you don't trust me and have some technical know-how, you can self host the service. It's open source: https://github.com/alexbakker/log4shell-tools.