English 101... For beginners.
Causal Words: a causal conjunction such as ‘because’ introduces a statement describing the cause of something.
The words "highest" and "lowest" are not causal words, they are adjectives:
Adjectives are words that describe or modify another person or thing in the sentence.
Furthermore, the words "highest" and "lowest" are the superlative forms of the adjectives "high" and "low", and their only function is to describe or modify the noun, or subject, of the sentence - in this case they describe or modify the word "imports".
Lets look at the sentence without adjectives,
"Imports are during booms and they are during recessions"
Like your entire argument, that sentence doesn't make sense.
Because I like humiliating people.
Imports increase during times of economic booms BECAUSE (that is a causal word) of an increase in domestic demand. An increase in imports is the effect while increased domestic demand for goods and services is the cause.
Imports decline during times of economic recession BECAUSE (causal word) of a lower domestic demand for goods and services. Again, the decline of imports is the effect brought about by the cause of lower domestic demand.
Now lets go back to your statement:
"[A]lthough a nation’s trade imbalance affects its GDP, the changes of that balance does not itself determine if the GDP will increase or decrease." - Supposn
You just said that; although the balance of trade affects GDP, the existence of an imbalance does not have the causal effect of driving GDP higher or lower. Then, above, you claim the exact opposite, that the balance of trade ALWAYS has the causal effect of driving GDP higher or lower.

Nope.