alt Hacker Newssqli is easily and fully mitigated and has generally been a non-issue for any half-serious project, especially if you use any kind of SAST. Your link actually subsumes any type of injection, not just sqli. Some of them are marginally harder to fix than sqli, most aren't.
In contrast, we don't know how to solve prompt injection.
Prompt injections are a whole class of vulnerabilities, and I would say there is generally a pretty good idea of how to mitigate them to be impactful. However in many cases those mitigations are not implemented (in the strictness that they require), as they are usually either too costly (second LLM as judge) or lead to worse UX (tool call confirmation with appropriate review of all input parameters on every tool call; disconnecting web access).