This shows an evident increase in the adoption of Bitcoin all over the world, although unique addresses does not necessarily translate to unique users of the network, it shows the growth of the network.
Is this evidence of Bitcoin going mainstream?
Would an ATH in addresses lead to a new ATH in the price?
No, it doesn't prove that bitcoin is going mainstream! Why?
The answer is already given by you - unique addresses doesn't necessarily mean unique number of people! And it certainly doesn't push for new ATH in price. With the increasing number of hacking incidents, it is always advisable to keep our bitcoins in different wallets and always create a new address while receiving payment from anyone! That's very basic security method we can follow to safeguard our assets!
However, increasing number of unique bitcoin addresses definitely means growth and adoption to some certain extent! Hopefully 2020 will bring cheers to our life when it reaches a new ATH!