Although this particular myth isn't advanced too seriously anymore, it hasn't been that long since people with considerable rank in the community were insistent it was true. Now, certainly there's a point where you don't really get good flavor out of a beer with too much "sugar" and nothing else to provide flavor but nobody really seems to know where that point lies. Just arbitrary numbers drawn somewhere. Personally I've brewed up to 20% of the fermentables from refined sugars added to the boil with zero cider flavor. I've seen recipes, even some older commercial recipes, that go as high as 30% with no reported cider off-flavor. So if there is a threshold where beer starts to taste cider-y just from adding refined sugars, nobody really has an idea what it is. I'll assert if such a threshold even exists, it's well above 50%.
When homebrewers talk about sugar making beer cider-y, they really mean refined sugars like corn sugar/dextrose or table sugar/sucrose. Most variants of the myth don't really explain how these cider flavors are created from dextrose or sucrose. I've seen some suggest the process of breaking down these sugars causes the off flavors so that's why you must invert the sugars first, but that's just more garbage speculation. If the enzymatic process of converting sucrose into simple monosaccharides yeast could consume was responsible for the off flavor than people making country wines adding sucrose as the sole or primary fermentable would always end up with a cider-y wine. They don't. The fact that they are able to ferment out a wine that is 100% or almost 100% sucrose should say enough to discount this whole stupid myth by itself.
If the access to too much simple sugar at once is responsible for this alleged off flavor produced by adding too much sugar in the boil, what about wines that are made from fruit that contain all or almost entirely all simple sugars like fructose? Again, no cider-y off flavor. Also, there's no scientific explanation that could be made why simple sugars produce off flavors but maltose and other more complex sugars, which are broken down to monosaccharides during fermentation, do not.
Maybe some component of beer is the distinction between this mythological off flavor and wines? Well, what about people who add honey to their beers, particularly braggots made of 50% honey? Honey is made of monosaccharides. So, again, no scientific basis is ever advanced why that form of simple sugar is ok but your grocery store table sugar is somehow different.
If you're getting a cider-y off flavor in your beer, the problem is in your process. You aren't treating your yeast properly (under-pitching, under-aerating, etc.) or you are fermenting too warm.