That’s not true. Winter tires grip just fine in warm weather. The only downsides of using winter tires in the summer are that they wear faster because of the softer rubber, have increased road noise due to the more agressive tread, and they’re just typically more expensive than all weather tires. Technically there is a slight handling difference because of the deeper more agressive tread but not enough that it is even noticable to most people. So it’s still a good idea to swap if you can, but they are not unsafe to use in the summer, just unwise for your wallet. If the vehicle you have them on isn’t a daily driver it may not even make sense to switch them out.