Question tag after a verb modified by "only"

When a verb such as "can" is modified by "only", the meaning of the sentence becomes rather negative (impossibility rather than possibility), and to me it sounds strange to use the negative tag "can't it?". On Google Books I find an example of an author doing it so,

It can only be Jagged Sky we're aiming for, can't it? (The Empty Sea, by Craig Michael Curtis)

But I'm not sure it's the most cautious literature. I would have said "isn't it", based on meaning:

It can only be Jagged Sky we're aiming for, isn't it? (= isn't it Jagged Sky we're aiming for?)

Is there a rule, and what is the actual usage?

EDIT: I understand that since "be" was present in the first example, my alternative suggestion is confusing as it seems to be related to it. A second example is:

Lorries can only use the main road, can't they?

where I wonder whether one would employ this negative tag or rather a positive one, "can they?" (="can they use other roads?")

Solution 1:

Lorries can only use the main road, can't they?

is grammatical, idiomatic, and is asking for confirmation of a fairly strong conviction. ('I think this is true; I'm correct, aren't I?'

Lorries can only use the main road, can they?

is equally grammatical, and uses a tag question either showing a degree of surprise at this newly disclosed piece of information, or sarcasm at a claim known to be false.

Sorry, no references on this one.

Solution 2:

The tag question negates only the active verb. Negating an already negated active verb results in a positive.

In the sentence, can is the active verb and "be" is an infinitive, i.e. not active, and thus "be" is not repeated. Thus, "can" becomes "can't/cannot" in the tag.

Compare:

It can only be Jagged Sky we're aiming for, can't it?

It is only Jagged Sky we're aiming for, isn't it?

See https://www.englishclub.com/grammar/tag-questions.htm