What does he mean by "crutch" exactly?
Marth is interesting in that he can basically be played well without a great deal of technical skill (he's easy to pick up), but requires extreme precision and spacing ability to do well at a high level (similarly to Jigglypuff; it could be thought of as a different type of techskill). He does still have lots of necessary and unique tech to learn (as does every character); check the Marth boards. Also watch this video for demonstrations and tutorials on Marth tech:
https://www.youtube.com/watch?v=_IACW5mltqY
Marth is also often recommended as a character for beginners to learn the game with; he will teach you fundamentals exceptionally well, even for a top-tier, nor will he harm your ability to learn any other characters at all. Learning spacies will improve your finger-speed though; you will be forced to learn to execute faster, though not necessarily play any faster. In a similar vein, it could be argued that playing Marth will force you to space better.
I would recommend you just do whatever feels right, since the value of how learning other characters helps your game overall relative to the time that could have been spent on your main is debatable. Personally, I'd experiment a lot, especially if you're new, until you feel right about settling; you seem to have already done that with Marth, which is fine. Anyway, you're not locked into the character forever; lots of high level players have changed mains, with examples from both before and after they were good enough to be noteworthy. On that note, Mango started off maining Jigglypuff (the least technical character in the game), and now he successfully plays a highly technical Fox and Falco (amongst the most technical charcters in the game).
I would also recommend watching lots of videos of your potential characters being played as technically as you can find. For example, the combo videos that come to mind for Fox and Marth are:
"Perfect Dark" (Dark)
"Version 2.0: A Mew2King Combo Video - Marth Edition" (Mew2King)
They are what you should aspire to in terms of tech. Also I should mention that Perfect Dark is the only combo video I've ever seen that starts with a techskill demonstration.