Are these guides still relevant or are they now outdated? With todays technology would a flagship smartphone and an external microphone do the same job as a dedicated recorder?
I have not seen a consistent smartphone setup that is able to do it. There are some options, with some stability issues, but its still better to just get a $100-$200 stand alone dedicated recorder. And then get external microphones.
Think about it. Recorder with built-in mics will always pick up handling noise, and the best you could do is put it in your breast pocket. Which for cardiod mics will get a great capture of you breathing down on it. External mics can be mounted on your head easily.