It’s not immediately clear, because the echo knight is not a creature, but some particular attributes are ascribed to it that are normally only ascribed to creatures.
There are several reasons for which we might state that the echo can occupy another creature’s space:
1. The Echo isn’t a creature
The basic rules impose the following restrictions on player movement, which are assumed to extend to all creatures, except in the case of exceptions:
You can move through a nonhostile creature’s space. In contrast, you can move through a hostile creature’s space only if the creature is at least two sizes larger or smaller than you. Remember that another creature’s space is difficult terrain for you.
The echo projected by an Echo Knight is not a creature, so this restriction does not apply to it.
2. The Echo doesn’t have a movement speed
None of the rules for movement, including the above, can be applied to the echo, because the echo does not ‘move’ in the way that creatures move. It has no speed of any kind, and moves ‘up to 30 feet in any direction’ when mentally instructed to do so by the Echo Knight. This movement includes upwards movement and is more akin to a caster moving their Mage Hand than to a creature expending their movement to traverse space in a physical way.
3. The echo is ethereal
Even if the Echo weren’t a creature and didn’t have a speed, it might still be constrained by the common sense adjudication that corporeal bodies cannot pass through one another freely. The echo, however is:
a magical, translucent, gray image
An ‘image’ is an ethereal object, which means that the echo can pass through creatures and objects alike.
All of the above is overwritten by the ruling that the echo:
… is the same size as you, and it occupies its space.
‘Occupy’ and ‘space’ here are not meant in their most general sense, but are mechanical terms usually reserved for creatures. So whilst the echo is not a creature, it inherits the capacity of creatures to occupy space, and the restrictions of space-occupation which come with them. Whilst these rules refer explicitly to creatures being unable to occupy one another’s space, they are extended to the echo because the echo is a space-occupier even though it’s not a creature. This is a difficult edge case brought about by poorly written rules.