It's a full-featured applications processor at MCU prices.
Just compare it to the $23.80 Teensy 4.0 which has 1 MB RAM (vs 64 MB) and runs a Cortex M7 at 600 MHz. That is definitely an MCU. And, when it came out, very good value for money -- I love mine. But the $3 Duo blows it away in pretty much every way.
If you want to talk about just the chip, the MIMXRT1062DVL6B in the Teensy goes for $14.48 qty 1, $9.17 qty 960.
According to the product page it actually even has an auxiliary MCU in it, in addition to its two high-performance processors.