The first person I ever heard refer to it as an M feint was Leonidas. It was a basic down block right swing outrange. The person who made the "how to M feint video" dueled Leonidas and I frequently. I believe around 2014-2015 I had some discussions with EU players/shared techniques (Soul and Lelastar) and by that time it was understood to call the technique an M feint because Tobi had many different moves but M had one very consistent feint (or what appeared to be very consistent).
This playlist right here was something I watched as a new player.
I turned on inverse to copy "Saulcanner" (In the above playlist, I am still not sure how their movements are so fluid except maybe movement key to block) late 2012. But I am not sure that the NA scene was as isolated from EU as projected in this podcast, in the peak of duel years competitiveness,(I would say 2015 for NA), nearly all of the top players in NA had a EU person they were copying and went so far as stealing faces sometimes.
After the switch to medium, the hilt-slash (which was common in EU already) and an "EU feint" became common in NA. An EU feint just meant a well timed block feint.
Anyway, if you are a duelist/ get nostalgia from this kind of stuff watch that playlist, lots of fun videos from both sides of the pond.
It's also interesting to hear the acknowledgement of how cliquey dueling has been at various points and you couldn't improve unless you could fight people who was willing to duel you enough to allow you to improve.