The author does imply that the test is effectively getting more difficult, but it seems like they're simply assuming that the adjustment must be happening in that direction without actual evidence.

In reality, because the test was normalized, it is useless for determining the change in academic achievement over time, and it could just as easily end up being normalized in the direction of effectively making the test easier. Because the scores can't be used for this purpose, other evidence would be needed to determine how achievement is actually changing over time (and therefore whether the normalization is effectively increasing or decreasing scores) and the author doesn't present any evidence that would show which is the case.

However, either way the decision to normalize the scores on a test that's apparently supposed to be used to show changes over time seems odd if the author is correct that normalized scores are being used for that purpose.