IMessage says delivered could that also mean they deleted my message?

Some of my friends have not replied to my texts in a few days, and it says on iMessage that the text is "delivered".

Could they have just deleted the message in their inbox before opening it, and that's why it still says "delivered"?

We all have iPhone.

No. Delivered means it was sent. It would still say delivered regardless of them deleting or not. What you want to do if you wanna know if they have read it, go to settings and it will have an option that will say something like show if it was read or something like that. And when they read it, it will replace the word "delivered" with "read (whatever time they read it).