MTG Myr Welder Exile question
Those two abilities are called Linked Abilities, which means the second one is referring to cards exiled by the first one. Specifically, rule 607.2a says


  If an object has an activated or triggered ability printed on it that instructs a player to exile one or more cards and an ability printed on it that refers either to “the exiled cards” or to cards “exiled with [this object],” these abilities are linked. The second ability refers only to cards in the exile zone that were put there as a result of an instruction to exile them in the first ability.


So, the second ability does in fact mean


  Myr Welder has all activated abilities of all cards that it has exiled.


There's nothing wrong with the wording, though. The fourth definition of the word "with" on dictionary.com is


  by the use of; using


In other words, they are saying that it has all of the activated abilities of cards "exiled [using] Myr Welder".