Raspberry PI's SPI over RS485
The single-ended  SPI signals can be converted to differential pairs, which are more robust in presence of EMI and can go the distance.  Then the differential pairs can be converted back to single-ended  SPI signals, and connected directly to peripherals (ADC, I/O expanders, and such).  That's what the original forum post is proposing.

MAX490 has 2 different sub-circuits.
One sub-circuits is a differential bus transmitter.  It can convert a normal single-ended SPI signal to a differential signal.
The other sub-circuit is a differential bus receiver.  It can convert a differential signal back to single-ended.

  (fig. 2 in the datasheet)


In order to connect the SPI bus this way, 3x MAX490 would be required on each side.  Something like this.



Clock delay

The above approach has a clock delay problem.  So it has limitation in bus length and bit rate.  This is what @Andy was referring to in his comment.

Here's an application note that further explores clock delay and proposed a solution (and this one too).  However, that solution requires 2 separate SPI peripherals on the microcontroller.

Here's another app note on clock delay in the context of high speed SPI.