It is extraordinary, but it also was incredibly hard to procure. Most data centres were almost immediately out of instances, and only the original batch of always free land grabbers really got these instances.
If you "upgrade" your account to PAYG you will not have this limitation. The "no free instances available" issue is only for accounts without a payment method.
I got this confirmed with a salesperson ~2yrs ago when I first set up my free tier on there.
Can confirm. I opened a new account and could never get any ARM instances in Ashburn region (capacity error). Signed up for PAYG (took a day - who knows why) and now no capacity problems.