Because this is science, not religion.

The thesis: Climate change is strengthening El Niño clearly uses the word "is".

Everything else is evidence, both for and against the thesis subject to interpretation.

Here it's "Something we can see in coral records correlates with what we'd expect given the thesis"

  The study doesn’t prove climate change is the culprit. But because the intensification coincides with rapid postindustrial warming and exceeds El Niño’s natural variability, the authors argue that human-driven warming is the leading explanation.

These observed correlations may be directly or indirectly linked to the thesis, or (and this does happen) they may be an unrelated coincidence.

As the weight of evidence in favour of a thesis builds, so does the confidence in the thesis itself.