This appears to be a targeted offer exclusively for United MileagePlus credit card holders. The email subjects says "Thank you -- Enjoy 2,000 bonus miles"

The 2,000 Bonus Miles Offer
  • Offer Link (Check your email if you are a United credit card member or fill out the form to determine your eligibility). 
  • If you receive the email, follow the Enroll link inside the email to enroll in the offer by September 15, 2018
  • Spend $4,000 on purchases from July 15 - Sept 15, 2018 to get additional 2,000 bonus United miles. 

Our 2 Cents
It is nice to see that Chase sends out an appreciate email to United MileagePlus credit card member for an opportunity to earn bonus miles. Hyatt also recently sent out an exclusive offer email to all Hyatt credit card member on getting 10% of redeemed Hyatt points back

If you just sign up for the United credit card, you can earn the sign up bonus after the minimum spend and stack up the spend to meet $4,000 on purchases to get this additional 2,000 bonus miles. But I doubt this exclusive offer is available to recently approved card members. We personally feel that the offer is pretty lame, especially when you have to spend $4,000 to earn merely 2,000 bonus United miles. If you have huge expense coming up and you don't have a credit card to spent that amount of money to earn the welcome bonus, this may be the way to earn additional United miles. Otherwise, I would not personally go out of my way to achieve this.

This offer has been added to the Points Promotions in one spot.