It appeared calculated to show China's naval reach . . .
It appeared calculated can be interpreted in two ways. You can interpret "calculated" as an adjective, meaning "it appeared to be planned and intended", which would follow the pattern "subject+appear+adjective". It's normally interchangeable with "it appeared as adjective".
The interpretation which applied here, though, was interpretation of "calculated" as a participle. "It appeared to be a calculation of China's naval reach . . . " In other words, you're right.
. . . is a not a combat vessel.
italic emphasis mine
Now this is more interesting. It reminds me of constructions like "an NARQ", which is a term previously used while moderating Stack Overflow questions. It stood for "a Not A Real Question".
Articles in English normally tend to appear as the first element in a noun phrase, before the nominal. There's the possibility of articles appearing in other places, of course. It usually occurs in colloquial language when using simpler but more words is preferable to using fewer but formal words people are less likely to understand.
Therefore, you normally see constructions such as "a getting-to-know-you meeting" instead of "a familiarization meeting" or anything that fancy. There are also more extreme cases where the hyphens are eventually removed and it's officially become a word, such as "whackamole". As you see, articles can very well appear in the middle of such words. We call those words compounds.
Colloquial speech, and sometimes formal speech (but not academic) is usually not restrained to what compounds it can use. The most important thing here is not having to explain what you just said.
So, in a bizarre context where "not a combat vessel" is a compound, most likely the 'collapsed' version of an acronym in a technical military context, it would be acceptable to say "a not a combat vessel", and the author would have "an NACV" in mind.
That's most likely not the case though, and this was just a typo that escaped the proofreaders (I had to read it thrice to notice it too).