It's silly to argue over the nuances of retcons vs. reboots, because these aren't terms that have any official definition to begin with. "Reboot" is a slang term, a figure of speech borrowed from computers, which in turn borrowed it from the expression "to pull oneself up by one's bootstraps," which is facetious and physically impossible.
In industry usage, the term "reboot" originally meant any revival of a dormant property, regardless of whether it was the same continuity or a different one. After all, the computing term just means restarting a program, not replacing it. But the term became popularized with the general public by the 2004 Battlestar Galactica revival, which was a full continuity reset, and so people came to associate the word "reboot" with that specific kind of revival to the exclusion of others, even though that wasn't how it was originally used in the industry (and even though it's pretty much the exact opposite of what the computing term means). Since then, fandom has gotten into the habit of using the word specifically for that, but that's just a popular vernacular usage, not some formal, legally dictated definition.
And of course, as with most things, there are gradations between different categories rather than absolute, impermeable dividing lines. Something like Kelvin Star Trek or the post-DOFP X-Men films represents a middle ground between a continuity revival and a continuity reboot in the modern sense; it serves the function of a reboot by making a fresh start that isn't bound to past continuity, but it uses time travel to justify the changes so that it can simultaneously present itself as a continuation, and thereby have it both ways.
As for "retcon," that's supposed to mean something that reveals new information about a past story without contradicting it, which is why it's called retroactive continuity. If a retcon does actually contradict the original facts, then it's a flawed retcon, or simply a deliberate inconsistency -- unless the original facts can be explained away as a deception or error (for instance, Obi-Wan telling Luke that Darth Vader killed his father). Ideally a retcon should agree with the established facts, just add new facts alongside them or reveal that they didn't mean what we thought they meant.