It was very difficult to find analysis let alone explanation of this poem so I apologise for any unsupported reasoning.
It is in trochaic tetrameter, because there are four trochees in the first line, as you highlighted in your copy (numbers indicate stress number):
TRO(1)chee TRIPS(2) from LONG(3) to SHORT(4)
With scansion symbols it looks like this (- = stressed, ˘ = unstressed):
There are four stresses to the line (resulting in a tetrameter) each with the stress on the first, not the following, syllable (resulting in trochaic meter).
Iambic tetrameter is the reverse of trochaic tetrameter as the unstressed syllable comes before the stressed syllable. If it was iambic, the stresses would look like this:
troCHEE trips FROM long TO short
If you try to say the line with the stresses like this, it is really awkward. The trochaic tetrameter just fits better.
(Using iambic stressing on the line would also only result in three stresses making it iambic trimeter.)
It is in fact the same rhythm and stress pattern and meter, as the lines from Macbeth:
DOUble, DOUble, TOIL and TROUble.
FIre BURN and CAULdron BUbble.
The stress is on the first syllables and there are four stresses to each line. However, there is an extra unstressed syllable at the end of these lines when compared to S. T. Coleridge's poem's line. Nevertheless, it is the stressed syllables which define the meter and the missing unstressed syllable at the end of the S. T Coleridge line therefore makes the meter catalectic trochaic tetrameter (a catalectic line is a metrically incomplete line of verse, lacking a syllable at the end or ending with an incomplete foot).
In the question you linked, it can be deduced that catalectic trochaic tetrameter and acephalous iambic tetrameter are somewhat difficult to separate; however, since this poem's form is meant to mirror the content, it is (catalectic) trochaic tetrameter.
The next line of the poem does not correspond with the first but corresponds with the third because is it describing spondees:
From long to long in solemn sort
The definition for spondees is: a metrical foot consisting of two long or stressed syllables, which the line is describing with "long and long". The second line is also separate from the first with a semicolon at the end of the first, so they are definitely unrelated. The real question is why is the second line not in spondees? I cannot find any explanations for this, apart from that the stressed 'o' sounds in the line act to emphasise the spondees in the next line. Also, in old copies of S. T. Coleridge's poems, where the lines are scanned, line 2 is sometimes left unscanned (along with the second part of the poem, which deviates away from describing meter and addresses his son), so maybe it is not meant to have a definite meter (sort does make a rhyming couplet with short so that could be the reason for this rogue line). Regardless of line 2, the line about trochees is definitely in catalectic trochaic meter.
