The key distinction
A layover can mean two different things: staying in the transit area, or clearing immigration to enter Singapore. SG Arrival Card matters when you are entering Singapore.
ICA states that travellers required to submit SG Arrival Card can do so within three days before arrival, including the day of arrival.
Visa and VFTF are separate questions
SG Arrival Card is not the same thing as a visa. If your nationality requires a visa, check official ICA rules before you assume you can leave Changi.
Some travellers may qualify for the Visa Free Transit Facility, but eligibility is specific and should be checked directly with ICA.
LayoverSG recommendation
If you are unsure about entry requirements, do not build a city itinerary around hope. Stay at Changi or Jewel until you have verified your status.
This is where truthful planning matters: the safe answer is better than the exciting answer if your next flight is at stake.
Next practical step
Start with airport-safe plans