Moral relativism doesn't mean what you think it means.

Moral relativism may be any of several descriptive, meta-ethical, or normative positions. Each of them is concerned with the differences in moral judgments across different people and cultures:

1. Descriptive relativism describes the way things are, without suggesting a way they ought to be. It seeks only to point out that people frequently disagree over what is the most 'moral' course of action.

2. Meta-ethical relativism is the meta-ethical position that the truth or falsity of moral judgments is not objective. Justifications for moral judgments are not universal, but are instead relative to the traditions, convictions, or practices of an individual or a group of people.[1] The meta-ethical relativist might say "It's moral to me, because I believe it is".[2]

3. Normative relativism is the prescriptive or normative position that, because there is no universal moral standard by which to judge others, we ought to tolerate the behavior of others - even when it runs counter to our personal or cultural moral standards.[3] Most philosophers find that this position is incoherent, or at least that it is unclear how meta-ethical relativism can lead to 'ought' statements.[3]

I'm pretty sure none of the above are relevant. The Israeli letter is mainly a complaint concerning the activists prioritization.