Possible to still get an absolute ring?

I wouldn't be surprised if this turns out to be the case, but the fact that the 10th day's potion made your 1st ring tradeable regardless of the upgrade status and the stamps status inclines me to think otherwise.

11th day and 12th day potions made your ring tradeable even if it was level 1.
How does this indicate you don't need stamps for the rewards?
The event description doesn't say anything about when rings become tradeable. But it does say quite clearly

- If players have participated at least 31 days, they can receive the Absolute Ring, which has the combined stats of all three Evolving Rings.
- Players can keep track of their progress by speaking to Gaga and selecting 'Check your event list.' This will bring up an Attendance Check UI that shows each date of the event, and stamps on the days that the player has participated in.

and

Receiving Rewards
Players are able to speak to Gaga during the Rewards Phase and earn a reward based on how many days they have participated/stamps they have collected during the Absolute Ring event. These rewards are:

Absolute Ring medal: 1 - 10 stamps: Category: Medal, Required Level: 13, Weapon/Magic ATT: +4
Absolute Ring Shoulder Decoration: 11 - 20 stamps: Category: Shoulder Decoration, Weapon/Magic ATT: +6
Absolute Ring Chair: 21 - 30 stamps: Recovers 50 HP every 10 seconds
Absolute Ring + all previous Absolute rewards: 31 - 33 stamps: Category: Ring, STR/DEX/INT/LUK: +18, MaxHP: +75, Weapon/Magic ATT: +15, Weapon/Magic DEF: +600, Accuracy: +6, Avoidability: +6, Diligence: +6, Speed: +15, Jump: +6