Of course it can fail savings throws
If it couldn't fail a saving throw, it wouldn't have specified the text you quoted:
If [the Echo] has to make a saving throw, it uses your saving throw bonus for the roll.
The fact that a situation is envisaged where a saving throw needs to be made, means it can make, and fail, savings throws.
Ok...but that doesn't answer my question
Your question's premise is that the Echo is not a creature. This is a frame challenge to that premise in your question. As argued in my answer to the linked question, the Echo Knight is a creature:
To start with, creature is not a strictly defined term in the game. Since D&D 5e uses natural language as opposed to strictly defined language, we need to figure out whether or not it "walks like a duck" so to speak.
But we have some clues it should be counted as a creature:
- It has an AC and Hit Point(s), so it can be hit and interacted with
- It has explicit immunity to all conditions
The second point is important, as conditions are described as (emphasis mine):
Conditions alter a creature's capabilities in a variety of ways [...].
If it's not counted as a creature, then it doesn't need to have the immunity specified, but it does, so it must be capable of being targeted by conditions. Ergo it is a creature, and is affected by all things that creatures are.
Since, it's a creature for the purposes of the rules, it can make and fail all the same saving throws a creature can.
But what if I don't agree that it's a creature?
Mechanically, for the purposes of fireball, it doesn't matter. The text of the spell also has this paragraph:
The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.
If it's not a creature, it is most certainly an object (since it has hitpoints). It is not being worn or carried. So, is it flammable?
It doesn't say it isn't flammable, and most objects are flammable, so being non-flammable is an exceptional thing, and thus would need to be specified in the rules as an exception, which it isn't.
Therefore, the Echo is flammable, and is automatically set on fire by fireball. Being on fire, causes fire damage, and the minimum amount of damage possible from being on fire is 1 hp of damage. Coincidentally, this is exactly the amount of HP an Echo has, and therefore fireball destroys any Echos that are in it's area.
But it says it's an "image"?!
Just because something is an image, doesn't mean it's not flammable. Nitrite film holds images and is most certainly flammable. A picture in a picture frame is an image, and is most certainly also flammable.