Good news, I was able to get the 100,000 bonus points as an existing AmEx card holder (Plat Edge). I had to ring up though. Here's how I did it (I applied 2 weeks ago or so so after when it apparently became harder to get the bonus points):
1) Apply for the card via a referral link (mine is
here). There's a good chance you'll see the offer with at the top right of the screen a LOGOUT button. Make a screenshot of this (just in case) since it shows that AmEx knows who you are and serving you up an offer for 100,000 bonus points. If they were to deny your points, that's great leverage to have!
2) Create a new American Express user profile online for the new card (don't use your existing login).
3) Check if the accounts are linked by logging in to your new American Express account online. If so, go to 4). If not, go to 5).
4) Ring AmEx to de-link the cards, thus creating a new Membership Rewards Gateway account for your new card.
5) Start spending, after reaching $1,500, wait a few days until all pending payments have cleared and see if points have hit your Membership Rewards Gateway account.
6) If your points have not been credited (as in my case), ring AmEx to ask for the points. If they question your eligibility (they didn't with me, they said they could see I was eligible), refer to the screenshot you have on file. In my case, the points were then added manually and only took 4 hours or so to show up in my account.
7) When you get your points, ring back again to re-link your Explorer to your Ascent membership (where appplicable) if you prefer to earn points at a 1:1 airline redemption rate for certain airlines.
8) Also ask them to combine your 2 logins in order for both cards and points to show up together while using the app!
I spent a fair bit of time researching all of this so would be grateful if you could use my referral link:
http://amex.com.au/refer/mellesHzBV
Happy to help with specific questions via a PM as well.