it's very possible to get DDoS'd using skype. several guides are available for the malicious. that's why discord has that in place.
You can ddos someone whether they're using Skype or not, all you need is their IP.
Also, they didn't say ddos, they said gank
Which, in MMO lingo (which is what this seems marketed towards), is when you're attacked by multiple players or one much higher level player, and you don't stand a chance against them. I don't see how Skype is relevant to that
Also I'm questioning the encryption claim they're making; you can encrypt the body of a message, sure, but IP addresses have to remain unencrypted, or the routers don't know how to route them. The only way I see around this is Tor, but I'd think that would be too slow for real time streaming, and
any slowdown in something as critical as gaming communication isn't worth tinfoil-hat level comforts. If you're worried about your friends grabbing your IP from Skype and ddosing you, get better friends