The specification of 10 knots as a delta is undoubtedly aircraft-specific (almost certainly based on the author's personal experience) and is not a prescribed number). The statement that "wet V1" may not be a "true V1" speed can be true, however....
It's important to understand the specific state of the regulations in force when any given work is published.
Today's FAR Part 25 explicitly requires that takeoff performance be provided for a wet runway. (25.105 at amdt 25-92, compared to the previous revision which only considered a dry runway).
The section on selection of takeoff speeds (25.107) which contains the stipulation that V1 have a relationship with Vmcg therefore didn't apply on wet runways prior to amdt 25-92. Strictly speaking, an OEM could have provided NO wet runway data prior to that date and been perfectly in line with Part 25.
The section on accel-stop distances also didn't worry about wet runways until amdt 25-92:
Originally Posted by amdt 25-92
Sec. 25.109 Accelerate-stop distance.
...
(b) The accelerate-stop distance on a wet runway is the greater of the following distances:
.....
(2) The accelerate-stop distance determined in accordance with paragraph (a) of this section, except that the runway is wet and the corresponding wet runway values of VEF and V1 are used.
amdt 25-42 was the previous revision to that section and contained no mention of wet distances (the corresponding para (b) is the new para (c), and so on)
Therefore, any "wet V1" derived prior to that date was, in effect, a non-certified number; it was basically advice by the OEM to the operator, with the FAA basically adopting a "three wise monkeys" approach.
So it is very possible that V1 was being scheduled without a strict regard to Vmcg in an older aircraft; it was a judgement call between the risk of a high speed abort - and going off the end of a wet runway - and the risk of an engine failure - and going off the side of a wet runway. There was an accepted degradation in safety for wet runway operations.