That article is appallingly inaccurate and pretty baseless.
IIRC, the 620 was not even released.
The 601 was the first in the PPC family, and started at 60MHz in the Apple lines. It was fast, the OS was still mainly 68k though, hence emulation slowed the system down. I don't know if Apple ever used the 602, but it was the chip between the 601 and the 603, obviously!
Yes, one wonders where the 68060 would have got to if Apple had used that processor. It would have made faster speeds for sure, maybe up to 200MHz in 1996/1997 time period. But the PPC was much faster than the x86 in 1998.
It is only the recent history of the PPC that it has lost any speed advantage - 1999 onwards in essence when the G4 failed to scale beyond 500MHz and the x86 processors (basically an x86 decoder with RISC core) scales upwards and onwards.
And why? Because Motorola don't care about desktop processors any longer. Embedded is their market. Without the money that Apple is putting into future PPCs, there would be very few upcoming fast processors. As it is, the ones that come out will be solely for Apples use for a long time, we won't see them in the Amiga market.