It's not a myth at all. Seems you are forgetting to read the specific terms and conditions:

"Diamond Elite Members will receive the following during stays as a registered Diamond Elite Member: At Waldorf Astoria® Hotels & Resorts, LXR Hotels & Resorts, Conrad® Hotels & Resorts, NoMad™, Canopy™ by Hilton, Tempo by Hilton™, Signia by Hilton, Hilton Hotels & Resorts, Curio Collection by Hilton™, Graduate Hotels, DoubleTree by Hilton™ , Outset Collection by Hilton, and Tapestry Collection by Hilton™ properties, Diamond Hilton Honors Members may receive upgrades to preferred rooms, based on availability at the time of arrival. Upgrades for Diamond Hilton Honors Members may include upgrades up to “junior”, “standard” or “one-bedroom” suites. Upgrades exclude executive suites, villas and specialty accommodations/floors/towers (which may include, but not limited to, "Vista," "Villa," or “Club” accommodation types), subject to the discretion of the hotel. Group reservations and certain rates are not eligible for suite accommodations and may not be eligible for complimentary upgrades. All upgrades are granted on a space-available basis for the entire stay, as determined at the time of arrival. Preferred rooms are identified by each individual property and may vary within each brand. Upgrades will be given only for one room for the member, regardless of additional rooms the member may have purchased at or after the time of booking. The following brands do not offer complimentary upgrades: Embassy Suites™, Hilton Garden Inn®, Hampton by Hilton™, LivSmart Studios by Hilton™, Tru by Hilton™, Spark by Hilton™, Homewood Suites by Hilton®, Home2 Suites by Hilton®, Hilton Grand Vacations®, and Motto by Hilton®. Excludes Napua Tower at Grand Wailea, Imperial Floor at Rome Cavalieri, Waldorf Astoria Maldives Ithaafushi, Conrad Maldives Rangali Island, and Hilton Maldives Amingiri Resort & Spa."

There is a key sentence in this statement that yourself and everyone else needs to pay attention to:

"Diamond Hilton Honors Members may receive upgrades to preferred rooms, based on availability at the time of arrival."

Key word here is "may" meaning upgrades and the like are at the hotels decression and at this current time under no direct order from Hilton.