Is it a case of explicit or implicit subjectivity in a poem if "the poet" is mentioned, as in the last stanza of Bill Manhire's poem, "Summer"? Last...

To
start with, let's define our terms.

• subjective/-ity: internal; personal; an
  individual's thoughts, moods etc
• explicit:
  clearly and fully stated
• implicit: indirect,
  not clearly stated

So when you ask if a poetic reference is
subjectively explicit or subjectively implicit, you are really asking whether the
poet clearly states their personal presence in the thoughts, ideas, moods, etc of
the poem (explicit) or whether the poet only suggests their personal presence by
implying an indirect, not clearly stated presence
(implicit).
 
Perhaps the best way to clarify this is to
compare Manhire's subjectivity to another poet's subjectivity and
see what the difference indicates.

A good poem to compare Manhire's to is
William Wordsworth's "A Few Miles above Tintern Abbey." In the first, second and final
stanzas, Wordsworth writes:

... Once again
Do I
behold these steep and lofty cliffs,
That on a wild secluded scene
impress
Thoughts of more deep seclusion; (1st)

.. Once again I
see
These hedge-rows, hardly hedge-rows, little lines
Of sportive wood run
wild (2nd)

We stood together; and that I, so long
A worshipper of
Nature, hither came ... (final)

Here, Wordsworth
clearly identifies himself as being subjectively present in the
poem with the first-person pronouns "I" and "we"; since he clearly
identifies his subjective presence, this is a case of explicit
subjectivity.

Your quotation is
the ending of Bill Manhire's poem "Summer." This is a metafictional poem that
discusses the creative poetic process while it is being undertaken: "Supposing this page is
a paddock / under snow ...." The last stanza speculates about what metaphoricthe words
being written on the page might represent. In this speculative or imaginative contemplation,
Manhire suggests the word might be a boulder for "the
poet" to sit upon:

[This]
word might be a boulder,
or ...
... a stone,
on which the poet sits,
somewhat alone,

Compare
Manhire's subjective reference to Wordsworth's subjective reference. While
Wordsworth clearly states his subjective presence with
"I", Manhire is less clear: Manhire
implies his presence with "the
poet"; he does not directly state his presence, rather he
indirectly suggests his subjective presence.

Our
conclusion, then is that there is a great difference between how
Wordsworth and Manhire approach revealing subjective presence. Therefore, if Wordsorth's direct
approach is a case of explicit subjectivity (and it is), then Manhire's must be a case
of implicit subjectivity. Is this true? Yes, it is: his
is implicit because he does not clearly state, rather he implies and indirectly
suggests his presence as "the poet."

Look at this from
the other direction for extra clarity: If Manhire had said "on
which I sit, somewhat alone," this would
have been explicit subjectivity like Wordsworth's, but he didn't, he said
"the poet," which is an
implicit expression of subjectivity.