I think you mean "despite the STM32 in the product name, they're 64 bit compatible cores." Because STM8 was 8 bit (though I've never seen one in any design ever, I'm sure they're just before my time) and STM32 MCUs and MPUs were always 32-bit.

I'm worried they dug too deep a trench with the 32. Like how do much from Espressif is an ESP32 now despite them ranging from slower single core to faster dual core, now even with mmu, some with Wi-Fi some not...

What is in a name?

STM32 processors have traditionally been M-series 32-bit ARM cores including M7. I don't know if there have been any exceptions before this. But A-series is application processors with MMU's, DRAM interfaces, etc.

STM8 is still a thing and they were popular cheap embedded boards before ESP32 took over. They were ok as a cheap alternative to Arduino-compatible AVR boards. I have a few of the boards around. They were around $1 each on Aliexpress and have 8K of flash and 1K of ram, which is enough to run interactive Forth, and there's an SDCC compiler target for them.

Or they could have just meant STM32 is normally synonymous with "typical slow single-core MCU" compared to multi-ghz Cortex SoCs that we see in higher-end devices that run Linux?

That's marketing. The iMac was a huge hit so Apple started releasing iEverything. Denny's had a hit with its "Grand Slam" breakfast now they have ten different "slams" on their menu,