Is it true that native English speakers can’t pronounce geminate consonants?

As other respondents have said, we do in word boundaries. I don’t do morpheme boundaries myself (I pronounce wholly and holy the same).

We *used* to have geminates, of course, which is why we have them still in spelling. That’s why -d- between two vowels only survives in native English words if it was a geminate, like ladder. If it was a single -d-, it turned into -th- : father, weather < Old English fæder, weder.