Per QA Team:
Tag applied: Not A Bug
Patron of the Nezumi's ability refers specifically to permanents. Permanents are spells that have been put on the battlefield, before they are, they are permanent cards or permanent spells. So milling cards that are permanent spells from the library does not count as being a permanent being sent to the graveyard.
The relevant ruling is not actually found in the Mill section of the comprehensive rules, but in the permanent types.
110.4. There are five permanent types: artifact, creature, enchantment, land, and planeswalker. Instant and sorcery cards can't enter the battlefield and thus can't be permanents. Some tribal cards can enter the battlefield and some can't, depending on their other card types. See section 3, "Card Types."
110.4a. The term "permanent card" is used to refer to a card that could be put onto the battlefield. Specifically, it means an artifact, creature, enchantment, land, or planeswalker card.
110.4b. The term "permanent spell" is used to refer to a spell that will enter the battlefield as a permanent as part of its resolution. Specifically, it means an artifact, creature, enchantment, or planeswalker spell.