I wish they set the author to the appropriate person. I assume Bill Gates.
from Paul Allen's Idea Man biography:
... It gathered steam with the release of the 6502 chip from MOS Technology at an unheard-of $25. For each new microprocessor, I created a new set of development tools on the PDP-10, while Ric [Weiland, 2nd new-hire, 4th employee] helped with the BASIC interpreter rewrites.
So Ric Weiland would have done much of the 6502 BASIC interpreter.
AUTTXT: ACRLF
12 ; ANOTHER LINE FEED.
DT"WRITTEN "
DT"BY WEILAND & GATES"
ACRLF
0> Rick Weiland and I (Bill Gates) wrote the 6502 BASIC.
I put the WAIT command in.
Mark Chamberlin and I wrote the 6800 BASIC.
Now who wrote the 6502 version? The KIM-1 BASIC manual credits Gates, Allen and Davidoff, the original authors of the 8080 version, but it might only be left over from the manual for the 8080 version. Davidoff, who worked for Microsoft in the summers of 1975 and 1977, had not been there when BASIC 6502 was written in the summer of 1976, but he probably changed the 6 digit floating point code into the 9 digit version that is first found in BASIC 6502 1.1 (KIM-1, 1977).
The ROM of the 1977/1978 Ohio Superboard II Model 500/600 (6 digit BASIC 1.0) credits RICHARD W. WEILAND, and the 1977 9 digit KIM-1 BASIC 1.1 as well as the 1981 Atari Microsoft BASIC 2.7 credit “WEILAND & GATES”. Ric Weiland was the second Microsoft employee. These credits, again, were easter eggs: While they were clearly visible when looking at the ROM dump, they were only printed when the user entered “A” when BASIC asked for the memory size.
According to apple2history.org, Marc McDonald (employee number 1) wrote the 6502 version, but it is more likely that McDonald wrote the 6800 simulator and Weiland ported 8080 BASIC to the 6800 and then McDonald adapted the 6800 simulator to the 6502 and Weiland wrote the 6502 BASIC.
This and the hidden credits in version 1.0 of 6502 BASIC suggest that Weiland was the main author of 6502 BASIC. Gates is added to the hidden credits in the 1.1 version, so Gates probably contributed to the 1.1 update..