I'm not sure what is bad science here. A link between oral health and physical health has been known for years. This is an observational study, observational studies are very powerful tools to help us figure out where to look. They are not saying A causes B or even that there is a correlation. All they are saying is that there is a coincidence of these two factors.
"...it may be that your oral health habits influence whether or not you get dementia," said Annlia Paganini-Hill
Look at the statement "it MAY BE that your oral health habits INFLUENCE whether or not you get dementia".
This is standard, good science to identify potential connections that will require further research to understand. There's a lot of bad science out there, this report is not one of them.
Here's another quote:
"Statistically, however, the effect was so small it could have been due to chance, the researchers said." -- imagine that, scientist admitting there is not a statistical correlation -- What bad science.