If Facebook doesn’t like this explanation and it circulates to millions of people, they are free to offer a competing explanation.
Arguing an unproven point isn’t a problem here, it’s a strategy, and it’s a good one.
A good guess for the actual reason would perhaps be that someone who posts lots of links runs a risk of posting ones found in a blacklist. That blacklist probably doesn’t deliberately contain “competing social networks” though, but could contain such links in error, indicated as phishing or malware.
However, given the strength of Facebook's monopoly, they have the power of a state over people's lives (e.g. via a livelihood that depends on a Facebook account).
The lack of due process (https://en.wikipedia.org/wiki/Due_process) is very worrying.
The Magna Carta says "No free man shall be seized or imprisoned, or stripped of his rights or possessions, or outlawed or exiled, or deprived of his standing in any other way, nor will we proceed with force against him, or send others to do so, except by the lawful judgment of his equals or by the law of the land."
This feels like a breach of that, even though it is a monopoly corporation, instead of a state.