That's not true at all, except from the standpoint of English. Plenty of other languages have consonant clusters that English speakers find impossible to say, but which somehow they do just fine with. How else do you think the Russians pronounce tsar or the Dneister and Dneiper rivers? How do you think the Germans pronounce the word zehn? How is it that Swahili can have initial sounds like Ng, Mb or Mv?
I'm not sure about Modern Greek, but the ancient Greeks had no problem importing a bunch of words with initial pt consonant cluster. It's not really that difficult to pronounce; I can do it, for instance. If I had a mike on this computer, I'd say it, record it, and upload it.