Plenty of really smart people believe in religion, too. It's a lot more complicated than just saying "Someone believes in X irrational belief, thus they can't be smart", especially since they easily could be really smart in other areas. Your views on vaccines, religion, and any number of other things really have very little bearing on how good of a software engineer you are.
Trouble is, people tend to forget they are domain experts. I trust my doctor for medical advice, but I don't consider them any more competent to dispense legal advice than a random acquaintance. However, it is common for highly-competent people in a domain to feel they are more qualified than average in other domains (which is a trait I actively work to control myself). Software devs seem very susceptible to this, since a large part of the field involves being able to quickly learn enough about a subdomain of knowledge to implement a required solution. I'm no expert statistician, but I have implemented multiple projects that required knowledge of statistics. However, I'm just at the point of "knowing what I don't know", while I haven't studied enough classical mechanics to truly grasp how much I don't know. So while I feel I have a good working knowledge of statics, asking me to design a bridge would be foolish. Sure, I may think I 'could' design a bridge, but that is just folly.
Yeah, 100% this. What's more, this is also quite common among actual scientists too. Quite a few creationists/intelligent design advocates are academics whose credentials are in fields completely unrelated to biology/evolutionary biology, but quite a few cranks/pseudoscience believers/theorists are in a similar situation.
This further reminds me of all the people who got degrees with me who weren't all that bright either.