It really depends on what you're doing, but there are certainly versions of Linux that are capable of hard real-time performance. But it shouldn't be too surprising that the goals of an operating system designed for multiuser, multiprocess context is not well aligned with the needs of real-time machine control applications. The trainer is correct, often times it really is appropriate to just delegate those requirements to a dedicated controller whose job is to do nothing else than keep performing the same DSP loop over and over in a well-timed manner. However, there are certainly cases where you might want to interact smoothly between real-time and non-real-time components of a system and use things like shared memory, and in those cases, there definitely are Linux-based solutions available. Like most things in engineering, there are trade-offs along this continuum, so knowing the options and the pros and cons so that you can make a good design decision is just part of the job.