Ok, I managed to get some info from my sources.

And yes, like I said before, I can confirm that it is one of their winback campaigns. They're targeting customers whose Astro accounts has been inactive for a number of years, and is still having some outstanding balance.
This usually happen when customer did not or refuse to pay Astro bills, and just let them "stop your service naturally due to non-payment". 
If that happens to you, then you won't be able to sign up new Astro account using your IC number anymore (even after many years later), until all outstanding balance is cleared.
This can be considered as "your IC number is being blacklisted by Astro".
Do note that if you do that, it will negatively impact your CTOS credit score, which can affect your bank loan, credit card, and other financial related applications, as it shows that you have a poor payment history.
By right, the correct way is, if you intend to terminate/cancel Astro subscription, even if you're broke and no money to pay, or even you're pissed of Astro due to some dispute, you still have to go through the proper termination process by contacting customer service, rather than stop paying Astro bill and let them "stop your service naturally due to non-payment". 

Now, this winback campaign allows these inactive customers to waive all outstanding balances, by paying a flat RM40, then it'll "properly close" the inactive account(s), and then you will be treated as if like a new Astro subscriber, where you proceed with Astro One pack with 24 months contract, and get new Ulti/Ultra Box as usual. In a way, it's like "start fresh".

As for how Astro reach out to those inactive account customers, they just use the mobile number associated with those inactive accounts, and blast WhatsApp message to those customers.
The thing is, since those Astro accounts has been inactive for a number of years, there is a possibility that the mobile number may no longer be associated with the original account holder (e.g. mobile number not up-to-date, account holder changed new mobile number, etc).
As a result, there is a chance that the WhatsApp blast message is being unknowingly sent to an unintended recipient, because it is believed that most customers usually won't think of updating their mobile number in Astro's system whenever they change to a new mobile number, especially after their Astro account become inactive.
Since the WhatsApp message contains the name of the account owner, which I assume you don't even know that person, I'd say you can ignore that message.
Btw can you provide a screenshot of that WhatsApp message? I'm curious to see how it looks like.

You can subscribe to HBO Max directly without going through Astro.

Astro only bundles it if you’re on their Movies/One Epic packs, or you can add it to your Astro bill (with RM1 off the retail price). If you just want to watch It: Welcome to Derry, subscribing directly and cancelling after works fine.

Are you subscribed to Astro? Which pack are you on now? Who knows maybe HBO Max already included and you weren't aware of it...

Mmm, so I guess you can't access to HBO channels like CH411, 412, 414, 415 right? In that case, then HBO Max isn't included in your subscription...

Since you plan to subscribe to HBO Max then cancel it after 1 month, you can get HBO Max on either methods:
1) Directly with HBO Max website, at retail price of RM34.90 / RM52.90
2) Addon through Astro billing at special price of RM33.90 / RM51.90
3) Addon through Unifi billing at much cheaper price of RM24.90 / RM34.90

If you subscribe to Unifi at home, I suggest you use MyUnifi app and subscribe HBO Max there, because that is the cheapest place to get HBO Max in Malaysia (that I know of).
Here's a full comparison with other streaming apps: https://forum.lowyat.net/index.php?showtopi...ost&p=111977955

Piece of cake. No penalty fees will be charged.

Plus, when you add HBO Max to your Astro bill, you’ll also enjoy access to HBO channels 411, 412, 414, and 415.

Yes.

For Astro, you can add and remove HBO Max addon at anytime via My Astro app or web https://myastro.astro.com.my/
The good thing of subscribe through Astro is as what doubleM2 mentioned, where you'll also get access to HBO channels on your Astro set-top-box, On Demand and Astro GO via CH411, 412, 414 and CH415.

For Unifi (be it Home, Fibre or Mobile), you can also add and remove HBO Max addon at anytime eithe via MyUnifi app, Unifi UniVerse app, Unifi Selfcare portal https://selfcare.unifi.com.my/ or Unifi TV sales portal https://unifi.com.my/login. 

As usual, when you subscribe to the app, charges shall be prorated from the day you subscribe. But when you remove the app, it'll only take effect on the next billing cycle.