A tutorial gives a sentence
will I get better results for my IELTS essay if I have interesting ideas
which uses the plural form of "results". However, there is only one essay task in an IELTS test.
Is that grammatically correct?
Yes, it is.
According to the Oxford Dictionary
1.3. A final score, mark, or placing in a sporting event or examination.
A panel of choreographers, artistic directors, and former dancers tallies these results into a final score.
Check the example that I have quoted: they tally results into a final score. Consider that your essay is evaluated from different points of view, different characteristics (plural) are checked - grammar, syntax, ... -, obtaining each of them a result. You receive multiple results from one essay and all results obtained are summarized in a final score.