Uh. DSP can be done on CPUs with no FPU and no accelerator whatsoever. This is routinely done.
You can do DSP even on an 8-bit CPU. Compared to many MCUs we used even 15 years ago, the RP2040, being a dual core M0+ (if I'm not mistaken) that can be clocked at up to 133 MHz, a lot can be done!
If you don't know how to do that, you can even use the CMSIS libraries.
Indeed, the FMAC thing is a recent feature found on the STM32G4 - a lot of other STM32s don't have this. Oh, and the FMAC is not FP either as far as I've seen - it's merely a fixed-point coprocessor of some kind.
It's all in whether a given CPU can fit given requirements. It's not at all in whether a given CPU embeds a particular coprocessor.
And with that said, it's hard to compare a G4, which is Cortex-M4 based with a FPU and various peripherals (and yes, a FMAC), with a RP2040, which is just Cortex-M0+-based, even if it's dual core. Two different worlds. Still, I bet a lot can be done on a RP2040 if used properly, I'm sure you'd be surprised. Of course don't expect the same performance. But why not compare it to a Cortex-M7 MCU with FPU, DSP instructions, and 1MBytes of core-coupled RAM while you're at it?