I encourage you to read that essay carefully. WP is a great resource. It is an amazing resource. It is additionally useful, helpful, unprecedented, and perhaps even world changing! I love it, that's why I volunteer my time here to improve articles and direct people to resources that can help answer their questions. But none of that makes it reliable, not in WP's own terminology. The scientific theory article is a great example. That's actually a pretty good article. And it's useful for many purposes. But the definition is presented too simplistically, and it makes it seem that all theories are generally accepted as true explanations of the world, and that is not the case, as is demonstrated by the phlogiston theory. And that example is confusing, but there are also many many inaccuracies and outright incorrect statements on WP  I know because I've fixed a few. Some are accidental, some are malicious, some will be soon fixed, but others linger. Another thing to remember is WP is always a work in progress. A true scholar should evaluate the reliability of every source. That is why I keep directing you to the SEP. That article is written by highly esteemed professional philosophers of science, who we know are educated, trained, credentialed and accepted in that community. Likewise, textbooks are vetted by editors and publishers, and academic journal articles go through peer review to help ensure that correct information is presented. That is part of why they are WP:RS. In contrast, WP is the encyclopedia anyone can edit. I can assert to you that I'm a professional scientist, with over a hundred citations in peer-reviewed journals to my academic articles, and a PhD in math, and maybe that will help convince you that I know what I'm talking about. But I'm not going to bother to prove that to you, and you can't really be sure that I'm not making that stuff up ) While we attempt to distill our reliable sources into good articles, sometimes that gets messed up. If you read the entirety of the SEP article on scientific theory, you will see that it does not support the sentence which cites it in our article! I don't know how that happened. But it will just sit there wrong until someone notices and has the time and energy to fix it. And even then, some well-intentioned editors may resist the change, and revert. And maybe it's not even wrong. Maybe some RS do reserve "theory" for widely accepted theories, and would instead say that we should rename the Phlogiston theory to "the former Phlogiston theory" or something else! So then we'd have to do a whole re-write, explaining several different notions of what a scientific theory is, and then it might make the article less useful for the casual reader. 
For 99% of lay use, the issue of reliability may not matter. And surely using WP's sources to cite in papers is a great idea - but you have to make sure that you read and understand them yourself, and don't depend on what WP says they say. If my life depended on it, I would not trust anything on WP, unless somehow I had no other sources available at all, and hence no choice. So no, WP is not reliable. But that doesn't mean it isn't great. Cheers,