The article mistakenly says "microSDXC slots" when it means "microSD" slot. Because SD, SDHC and SDXC all have the same physical form factor and slot you put them into, but vastly different electrical interfaces and protocols.
And yes, older versions like SD literally did not have the bits to even communicate cards with 512 GiB.
SD, SDHC, and SDXC are all electrically the same (though newer cards can also optionally support higher-speed interface variants with different signalling voltages), it's just that the original SD protocol didn't have enough bits in the protocol to support higher capacities. SDHC and SDXC are basically the same except that one uses FAT32 and the other uses exFAT as its standard format. You can often use cards above the official 32GB limit in SDHC devices if you can format them as FAT32. A lot of older SD card slots can also be made to support SDHC with a suitable firmware or driver upgrade because the protocols are really similar, but with some fields just being interpreted slightly differently.
And yes, older versions like SD literally did not have the bits to even communicate cards with 512 GiB.