NOT TRUE ... as long as all four tires are identical (same brand, model, and size) and the measured circumference of all four tires is within 1/4 inch (or measured tread depth within 1.3/32 inch). It should go without saying not to mix summer and winter tires (i.e. on the same car at the same time); it has to be one or the other.
FWIW, both 225/50R18 and 225/55R17 were OE tire sizes for MY 2015-2019 Legacys, depending on vehicle trim level. The 17 and 18 inch OE wheels and tires are interchangeable, as long as all four match. If in doubt, consult the Owners Manual.