“it says איזהו חכם הלומד מכל אדם like Rebbi Meir who was able to learn even from Acher.“
That’s true however tosfos gives a reason why Rav Meir can do it, there are Halachos regarding who one can learn from, and they make a differentiation between a גדול & קטן and if a person will be swayed by the apikorus then he’s a קטן and it’s not so pashut who won’t be swayed