I don't much care for the very high strand counts. It is flexible but wears much easier. The resistance of a 1ga wire over 10-12ft is pointless to argue over if you are using copper. They are essentially the same when talking about this application.
Anyways; heres some math/physics for the nerds
For No. 12 AWG Table 8 of the NEC does list two different DC resistances for
stranded and solid copper.
DC resistance per 1000 feet for solid is 1.93 ohms
For stranded the DC resistance is given as 1.98 ohms per 1000 feet.
So solid should have a slightly higher ampacity.
If we use Table 310.16 of the NEC to determine RCA and substitute into the
Ampere calculation we can find the approximate differences in ampacity.
From Table 310.16 using 75 degrees C as the ambient.
I = 25 amperes, TC = 75 degrees C, and TA = 30 degrees C and RDC = 1.98 ohms
per 1000 feet or 0.00198 ohms per foot.
This converts to 1980 microhms.
From I (in kiloamperes) = SQRT(( TC-TA)/RDC*RCA))
Or RCA = (75-30)/1980*0.025*0.025
RCA = 36 thermal ohm feet
For stranded, I = 0.025 kiloamperes from the table
For solid No. 12 copper
I (in kiloamperes) = SQRT ((75-30)/1930*36)
or I = 0.0254 kiloamperes
Then the solid No. 12 copper would have a 0 .4 ampere increase in ampacity.
This is a 0.4/25 *100 or only a 1.6 per cent increase.
Considering that ampacity tables are approximations, this increase in
ampacity does not exceed the error of approximation.