I say that advice is rubbish.
A proper reputable summer tire will outclass a winter tire ANY time during the summer.
This is not anecdotal but based on several tests and reports from government agencies and private magazines/programs over the last decade.
Google "using winter tires in summer" for a small sample of why your dealer is full of crap. Most reports is for car tires obviously, but the laws of physics and rubber compounds are the same.
That being said a winter tire is better when the temperatures are lower than what you would normally associate with motorcycle riding (<5 Celsius).
Can you ride with winter tires in summer? Yes, sure. Is it a good idea? No, it isn't.