It states to not leave the vehicle exposed to those temperatures without it being plugged in, which, to me implies that the car will automatically heat the battery to prevent the electrolyte from freezing.
The manual doesn’t state that regular user intervention is required in extremely cold temperatures. I could be wrong, but what’s the point of plugging it in if it doesn’t do anything?
I could not find the statement to not leave the vehicle exposed to those temperatures without it being plugged in. What the Tesla Model Y Owner's Manuals states, seems to be concerned with, is leaving the Tesla Model Y vehicle unplugged for extended period where the battery may become fully discharged.
Page 154:
Battery Care - Never allow the Battery to fully discharge. Even when
Model Y is not being driven, its Battery discharges very
slowly to power the onboard electronics. The Battery
can discharge at a rate of approximately 1% per day,
though the discharge rate may vary depending on
environmental factors (such as cold weather), vehicle
configuration, and your selected settings on the
touchscreen. Situations can arise in which you must
leave Model Y unplugged for an extended period of time
(for example, at an airport when traveling). In these
situations, keep the 1% in mind to ensure that you leave
the Battery with a sufficient charge level. For example,
over a two week period (14 days), the Battery may
discharge by approximately 14%.
Discharging the Battery to 0% may result in damage to
vehicle components. To protect against a complete
discharge, Model Y enters a low-power consumption
mode when the displayed charge level drops to
approximately 0%. In this mode, the Battery stops
supporting the onboard electronics and auxiliary 12V
battery. Once this low-power consumption mode is
active, immediately plug in Model Y to prevent a jump-
start and 12V battery replacement.
Temperature Limits -
For better long-term performance, avoid exposing
Model Y to ambient temperatures above 140° F (60° C)
or below -22° F (-30° C) for more than 24 hours at a
time.
https://www.tesla.com/sites/default/files/model_y_owners_manual_north_america_en.pdf