I've never heard those bits referred to as 'CoS bits' before, when someone says CoS, that to me means the IP layer QoS field, the COS, TOS, PREC, DSCP field depending on which set of standards you want to follow. The VLAN bits are always referred to as Priority bits or P-bits.
Here's how Wendell Odom explains it in the SWITCH courseware:
—Each frame is tagged with a 12-bit VLAN ID and a User field. The
User field contains three 802.1p priority bits that indicate the frame CoS, a unitless
value ranging from 0 (lowest-priority delivery) to 7 (highest-priority delivery). Frames
from the native VLAN are not tagged (no VLAN ID or User field), so they receive a
default CoS that is configured on the receiving switch."
I think the dude in the linked thread conflated the two.
(I can't bag on the guy, I totally flaked that dot1p was different from dot1q...)