This is false FUD that keeps being repeated. It's not true. No iPhone ever has had a baseband with DMA access to my knowledge, and modern Qualcomm devices have advanced IOMMU systems to firewall away the baseband from the rest of system memory. I'm sure some phones somewhere existed where the baseband was privileged, but it's not the norm.
Companies like Purism keep repeating this lie (their marketing is outright false) to sell you less secure "free" phones that actually have a larger attack surface for the giant proprietary baseband blob. On the Librem 5, the baseband is connected via USB, and they don't have USB device filtering enabled, so the baseband is exposed to the entire Linux kernel USB device driver attack surface. We know that's full of exploitable vulnerabilities, just ask any USB device developer how many times they've run into a kernel panic by accident. That's much worse than an embedded baseband with a single purpose shared memory interface and proper DMA restrictions.
Please research this stuff before continuing to propagate this myth. It doesn't help users' freedom nor security to have it being parroted over and over again.