That's *NOT* dual sim active. There is only one single radio in your phone. You're relying completely on carrier #1 to allow the VoLTE/IMS traffic for carrier #2. If carrier #2 blocks data to carrier #1 for whatever reason, this will not work.
This is Dual Sim Dual Standby with a workaround thanks to VoLTE.
I know what you mean, but because this feature is brand new and no official body has decided to differentiate the two, we can call it dual-SIM active. For example the
GSMArena glossary says:
Dual-SIM Active (DSA) enables both cards to receive calls at the same time. This feature usually requires an additional transceiver for the secondary SIM card, and as such consumes more battery life. More recent models feature Dual SIM Dual Standby (DSDS) technology which enables them to have two active SIMs with only one transceiver.
Emphasis on the word usually is mine.
Blocking of the IMS traffic might be a US problem: in countries like Canada, carriers do not have the right to block traffic (neutrality of net).
And like I said, this method of doing DSA will probably become the norm because no manufacturer wants to implement two radios in mainstream phones in 2019, with all the drawbacks like battery life.
By the way, you can always switch which carrier is carrying the data for the other carrier's VoLTE. So in terms of reliability it's not really an issue because you can user either data line.