I wouldn't read that much into this. Here's the tweet in question with a video:
x.com/LindseyStirling/status/917275750746570752
At the end, someone picks Lindsey up by her waist and she kind of squirms and shouts but it's hard to tell if it's a ticklish reaction or if she was just surprised at being picked up. This Dan Sortin guy doesn't appear to be someone who would actually know from experience that Lindsey is ticklish, but rather just a fan of hers who watched the video and assumed she got tickled when her waist was grabbed.