“Android App Support” is AlienDalvik! It is just an non-technical / marketing name for it.
Jolla introduced that marketing name some time after they had taken over the development of AlienDalvik from Myriad Group.
It is build upon AOSP code (Android Open Source Project), which is also what LineageOS and other Android distributions (“ROMs”) use.
And it comes in different versions, which (mostly but not only) depend on a minimal Android vendor kernel (i.e., an adapted Linux) version used.
It is the specific Android vendor kernel used by Jolla, which prevents the use of AlienDalvik 8.1, 9 and 10 on all devices older than the Xperia XA2.
Just as the the specific Android vendor kernel Jolla adapted for SailfishOS on the Jolla 1 phone (from Qualcomm) prevented Jolla from deploying AlienDalvik 4.4 on it, while the Jolla C / Intex Aquafish, Jolla Tablet and Xperia X use that AlienDalvik version.
Thus, if you want a newer AlienDalvik on any of these old devices, you first have to obtain an sufficiently new Android vendor kernel for them, which supports at least Android 8.1 (because no AOSP 5, 6, or 7 based AlienDalvik exists).
Then you have to adapt that kernel and some low-level parts of SailfishOS to run well together.
Side note: IIRC Qualcomm provided a sufficiently new Android vendor kernel for the Jolla 1 hardware to run Android 4.4 and Sony a sufficiently new Android vendor kernel for the Xperia X to run Android 8.1, though Jolla has not taken the effort to adapt these kernel sources for the use in SailfishOS: Adapting and testing takes quite some manpower and Jolla rather put their slim resources into adapting the vendor kernels of newer devices, which is absolutely the right decision from a business perspective.
But it is all Free Software, so you can start doing this right away, if you want.
TL;DR: No, Jolla is not cheating, IMO you should not confuse marketing and technical names (take a look at the RPM name: It always was & is “aliendalvik”).
And they are not artificially holding back newer AlienDalvik (“Android App Support”) versions for older devices, they just have no incentive to perform the necessary work to make this happen.
P.S.: And as these old devices are old (specifically their battery are worn, but they also use outdated hardware etc.) my advice is: Do buy a newer device!
P.P.S.: Although I do own a Jolla 1 and an Xperia X and I would love to install and use a newer kernel and a newer AlienDalvik on them, I do not want Jolla to put significant resources into that! I rather would like to see Jolla performing more thorough quality assurance, so SailfishOS becomes less buggy. From my perspective they have created more and more serious bugs with each SailfishOS release since 3.2.1 than they fixed, rendering all these releases unsuitable for my real life usage (though I have some hope the 4.3.0 may be good enough).