Even free verse can include an occasional rhyme. The key is its metrical irregularity and avoidance of being defined as a fixed form.
So what makes this poem interesting is that is appears to have regularity, because of the repeated 4-line verse paragraphs, which look like a regular stanzaic form. But the unpredictable meter contradicts this appearance, giving us two opposites at the same time: regularity and irregularity.
The poem's form is mirroring its content: the world is "collateral and incompatible" and "Incorrigibly plural." It doesn't fit a single category: winter and summer, "spiteful and gay," all at the same time. Opposites, just like the poetic form.
I don't know MacNiece's work, but this is a lovely and intriguing piece, and this is how I make sense of it.