Hi willmwade,
I don’t have any personal experience with their mattresses (although I do know they don’t have specific edge support) but there are some general comments about edge support with foam mattresses in post #33 here.
In “theory” there shouldn’t be an issue with thinner mattresses (like their 5" model) and if a thicker mattress also has a firm support core then there normally wouldn’t be any issues either although some people who sleep with the parts of their body that have a higher weight concentration on the outside few inches of the mattress may notice some “roll off” on the very edges with some mattresses.
Tuft & Needle would be able to tell you more about their customer’s experiences with this (and they would tell you their actual customer’s feedback rather than what they “wanted you to believe”) but hopefully some of the members here who have purchased one will also see your post and can “experiment” with sleeping on the very edge of their mattress and share their thoughts and feedback.
Phoenix