This story is likely bogus.
Internal polling would not vary all that much from public, because they turnout models can’t be all that different.
And there is the reality that Trump’s win aligned with most polls +/- MOE. The polls were pretty accurate.
Remember right before Brandon withdrew from the race that there were a bunch of polls that seemed too good to be true but maybe they were true. I definitely remember seeing polls either showing Trump leading or very close in VA, NH, NJ, MN, NM, CO, and even OR. That would get Trump to around 380. Trump certainly had the momentum and Biden was apparently taking on water at a huge rate, so some other Democrat states could have become vulnerable. IIRC, there was a poll from Illinois around that time showing Trump only a few percentage points behind and that would get him to about 400 EVs.
Brandon's withdrawal and Harris' appointment reset the electoral map and erased most of Brandon's deficient. IMO Trump was starting to pull away at election time as the electorate was finding Harris unacceptable. A couple of more weeks and Trump is probably in the mid-300s.