It looks like quite some people disagree with my point of view, so I'll expand further.
It looks like spam, but it shouldn't be treated as spam
I assimilate these posts as intentional gibberish, which, according to the offical rules, should not be flagged as spam, but rather as "rude or abusive" (which works almost the same as the spam flag).
Excerpt of the official rules regarding "spam" flagging (emphasis mine):
A post should be marked as spam only if it advertises a product, service, or similar and is unsolicited or lacks disclosure.
It should not be marked as spam when
- The post contains no useful information, such as an answer that says “I don't care about your problem”. Flag as not an answer instead.
- It contains only gibberish, such as “fsdguejgkfdlk”. Use the rude or abusive flag for these cases.
And regarding the "rude or abusive" flag:
A post should be marked as rude or abusive (formerly known as offensive) if it contains hate speech, obscenities, abuse against people, or abuse of the community or system, i.e., a clear violation of the be-nice policy.
Abuse of the system or community is everything that is created with the intention to harm them. This includes posts that contain no useful content at all – i.e. gibberish posts along the lines of:
Both these flags work almost the same anyway, so it doesn't matter much. They will both lead to the post destruction, and a -100 rep penalty for the author (and potentially his permanent ban in the long run).
There are, however, two differences:
If a question gets two answers that are flagged as spam and deleted, it will be automatically protected. The same doesn't happen with rude or abusive flags.
If a post has helpful spam flags, then it can be used as an audit in review queues. If the post has only rude or abusive flags, then it won't be used.
This is where flagging as "rude or abusive" makes sense rather than "spam":
The gibberish posts are typically sent under random questions, so there is no reason to automatically protect such questions if they have been unlucky enough to get two gibberish posts as answer. On the other hand, real spam is typically sent to targeted questions, whose subject is more likley to attract spam (I have regularily seen spam posts from PCB fab houses on question about PCB manufacturing, for example). Automatically protecting these questions makes sense.
The gibberish posts are too easy to identify, so it does not make sense to reuse them for audits.