"The choice of of forms something close to a grammatical phrase ("an array of five ints")."
That's not how "of" works, though. It doesn't pluralize like "x" does.
"5x integer" means "5 integers".
"5 of integer" does not mean anything.*
The key here is that the x needs to be a suffix, and have whitespace after it.
"5 x integer" doesn't do the job.
This commonplace shorthand, [5x Int], has been brought up at least twice, but not addressed: SE-0483: `InlineArray` Literal Syntax - #383 by benrimmington
* [5 of Int] is learnable as broken English, however, and while it's worse than [5x Int], [5, Int], and [5-Int], it's better than [5 x Int], and I'd use it if [5x Int] doesn't work, like those other ones don't. I feel like it's better to recognize how weird of is, by spelling it like this, though: [5 o' Int].