Huh, would it be possible to provide a source? I might be bad at searching, I'm not finding anything...

EDIT: Ok I found one with some search operators. I can provide links, most were less trustworthy, I'd reserve judgement.

1. An organization which was initially responsible for Matrix, AMDOCS, is allegedly (I say allegedly since I didn't confirm it to a reasonable extent) an organization based in Israel which appears to have products related to surveillance
2. By association, Matrix is tainted, perhaps it has sophisticated backdoors along with the other myriad of issues mentioned by other commenters

To give an alternative explanation with plausible hypotheses

1. An organization linked to intelligence surveillance, created and discarded software, which occurs with most software, and I would imagine occurs with software developed at an organization linked with surveillance as well (if it's publicly funded, i.e. by a government, I'd lean into this)
2. Though suspect in origin, the amount of time the software has been independent, and with its open codebase, means any backdoors or other nefarious artifacts can be reasonably said not to exist
3. An organization linked to an intelligence agency would perhaps be the one to expect to have a secure messaging platform, one could imagine said organization would develop a solution in-house as even with software audits, they may not be certain of any external software which may itself be compromised by an antagonist or have vulnerabilities which they could not control

Some food for thought. I'm not one to jump to conclusions, I think claims require proportional evidence, and obviously my judgement isn't the same as a security researcher or clandestine operator, so settling on what 'appears' to be true without proper investigation isn't something I do.

Thanks for the info though!!