I think that technically she did [break the rule].
The rule says "directly or indirectly" and "by any form of words". This would
include quoting a letter.
Okay, what do the rules say? Lets see:
2. No Senator in debate shall, directly or indirectly, by any form of words impute to another Senator or to other Senators any conduct or motive unworthy or unbecoming a Senator.
See? Elizabeth Warren didn't break any rules. The letter Elizabeth read in no way said directly or indirectly that Jeff Sessions is unworthy of being a Senator. The letter from Mrs, King merely said that Senator Sessions has shown himself unworthy of being a Federal judge. There are different criteria for the two positions.
A Federal judge has to prove him or herself to be capable of judgment, sobriety, legal knowledge and respect for legal traditons. A Senator is elected by his own state for whatever reasons they choose, and there is no requirement at all that he or she be capable of judgment, sobriety, legal knowledge or respect for legal traditions. In fact, in order to be seated as a Senator about the only requirement is that you not be not obviously engaged in multiple felonies at at the moment and not be clearly mentally ill or intoxicated. Otherwise, what's good enough for the voters back home is good enough for the US Senate.