In my experience peer reviewed proceedings are okay but if its a partner journal its not. I actually have fairly recent experience with this because one of my papers that I just submitted to a journal was published at one conference as a part of peer reviewed conference proceedings. Since, I am young scholar myself I also first consulted this with both my mentor and I actually even send an email to my target journal asking if submitting paper to that conference counts as prior publication that would disqualify it from publishing there and they said its okay they dont count that as prior publication. However, it would not be okay if the publication would be in actual journal (if the conference has a partnership with one) - the website you linked mentions this.
But in case the conference is partnered with some journal and this is not just proceedings they wont publish there your work without your consent - at least that is my experience. Last summer I send one other paper to a conference organized in my home country and they were also having similar opportunity as the conference you linked here and actually they selected my paper to be in the journal but since its a local publication I declined and they were fine with it - at least to my best knowledge (although probably try to first send an email to the conference organizers).
Also when I was inquiring about this generally more experienced professors told me its usually positive to have your paper been already in peer reviewed proceedings. They told me that reason for that is that most journals dont have double blind peer review. Hence the reviewers can actually google you and some might think that if your paper was accepted at a good conference and already been peer reviewed once then it is of high quality, although I suppose the converse could be true with bad conference. This being said I only have two papers under my belt so maybe take this advice with a grain of salt, but I think the advice of the more experienced professors at my uni should be solid, some of them have quite impressive publication records.