Aid is treated as a level 1 spell for the Ranger
A level 2 spell uses a level 2 spell slot.
Normally when you cast a spell, you expend a spell slot equal to that spell's level or higher as per the rules for Spell Slots:
When a character casts a spell, he or she expends a slot of that spell's level or higher
The Ranger's Spellcasting feature gives a description of how they learn spells:
The Spells Known column of the Ranger table shows when you learn more ranger spells of your choice. Each of these spells must be of a level for which you have spell slots.
That is to say, a level 2 spell can be learnt when you have level 2 slots available and is cast by using a level 2 slot. These are the established rules.
Rules have exceptions
One of the core tenets when reading D&D rules is that "the specific overrules the general".
In this case we have a level 2 spell added to the Ranger's spell list as a level 1 spell. We are going to assume this isn't a mistake. What does it mean?
Firstly, since it's listed as a level 1 spell for Rangers you only need to have level 1 spell slots available to learn it.
Secondly, the spell list defines spells and their levels. This seems to indicate that even though the spell is level 2, it is treated as a level 1 spell for the class.
This second point is further supported by considering the implications of the first. If a class has no level 2 slots available, how will they cast their level 2 spell that the rules are specially written to allow them to take? In this case the intention of the designer certainly seems to be for the class to treat the spell as if it were a different level to normal.
Even if that is the intention, it's messy
As you note, this is probably a mistake. The wording leaves too much to be questioned; for example if a Ranger uses Aid, is it still treated as a Level 2 Spell for other purposes? A single sentence such as "the Ranger can learn and cast Aid as if it were a level 1 spell" would have cleared up all problems.
That said, plenty of problems exist in D&D, so this could be the intention of the designer, but it was just poorly worded and yet another problem.
Although, the existence of Tireless might mean that early access to Aid was intentional. It's hard to guess the intention of the designer, so you are probably best with RAW or talking to your table!
(This is one problem with "trying to guess the intention of the designers": if a rule says X then it does X. There is no point in trying to second guess everything in the rules just in case there was a mistake. Assume the rules are correct unless an official source corrects them. You will not get very far trying to understand the rules by questioning if the rules are correct or not. They are, and even if they don't make sense, they are still the rules. Nothing is forcing you to follow the rules, if they don't make sense then make your own.)