The echo is not a creature.
The echo is defined as an image, and nowhere in its description as a creature or object if not only by some degreee of inference. If the echo was a creature, the feature manifesting it (creating / summoning) would explicitly tell you that game detail, as most other already existing effects do.
There are many effects, specifically illusion spells and similar ones, capable of creating an image which can look like a creature, yet there's nobody denying that that image is just that. There are illusions that can have an AC whilst being just illusions, like those of the spell Mirror Image, disappear when taking damage, like the ones of the spell Project Image, or act as a proxy for the caster to perceive / take actions through it.
To name a few spells that create / summon creatures and go out of their way just to tell you that this new thing is, in fact, a creature (emphasis mine):
• Animate Objects:
Choose up to ten nonmagical objects within range that are not being worn or carried. [...] Each target animates and becomes a creature under your control until the spell ends or until reduced to 0 hit points.
You shape an illusory duplicate of one beast or humanoid that is within range for the entire casting time of the spell. The duplicate is a creature, partially real and formed from ice or snow, and it can take actions and otherwise be affected as a normal creature.
• Other spells which provide a stat block (something unique to creatures) and still define the newly created / summoned thing as a creature are Animate Dead and Create Undead, all the Conjure spells (e.g. Conjure Celestial or Conjure Minor Elementals), and all the other summoning spells (be the demon/devil summoning from Xanathar's Guide to Everything or the Summon spells from Tasha's Cauldron of Everything). The same logic applies to the special companions gained by Warsmith Artificers or Wildfire Druids (or animated by a Creation Bard) via their class features.
To clarify, there are also spells and features that create objects (or force stuff) that can have some similarities to creatures, yet those still include a line to define this new thing as an object (or force). Some of those spells are Bigby's Hand and Unseed Servant, or the Eldritch Cannons created by an Artillerist Artificer.
If the second tweet marks the intended function of the feature and the echo should behave like an object, it can be ruled as such until an errata of Explorer's Guide to Wildemount or a sage advice about it comes out and gives us stronger confirmation. But reading the feature as it is worded now, it is not an object (or at least it isn't clearly one).
If a game effect does not define the new thing as a creature or an object, it is neither. Mage Hand, for example, creates a spectral floating hand, and the spell Mordenkainen's Faithful Hound conjures a phantom whatchdog that is just a spell effect and not a creature.