I've been on MangaToon for a few months now? Like @ghostmaya I emailed them directly to publishing under them (non-exclusively). As far as I am aware, you do keep copyrights of your own comic/IP if you sign non-exclusively.
However, since I am not an expert on legal rights or MangaToon itself, I will say this. I have a hard time researching this too because there's not much info on it, so I don't know if they have control over your IP rights other than what is stated in the document, but I would say go in knowing that there is a possibility of them holding (at least some) rights. Be aware and know that when you aim to sign into any legal deal with a publisher or company, there will likely be that possibility of the other party holding some sort of right over your IP (whether to post on their platform, give you payments, etc) because that is how a business deal works. It's a negotiation between parties (and usually to the company's benefit), so always go in aware of that fact and make sure you negotiate your rights and can agree to the other party's terms before giving your signature away. (it's also good to make sure there is a termination clause)
I don't want (nor am I trying) to discourage anyone from signing on to MangaToon (or any publisher for that matter). It may work for you, it may not. It's different for everyone. Everyone seeks the best of their comic (whether it's with a publisher, self-publishing, etc) it's up to you to decide. I just don't want anyone to go in blind to a publishing company without knowing what they're playing at.
Ofc, if there's any other questions (either abt MangaToon, WebcomicsApp, etc), feel free to hit me up. I'm always happy to help ^v^