Seems like nodeID is forced to convert to string at https://github.com/ezsystems/ezpublish-legacy/blob/master/kernel/classes/datatypes/ezxmltext/ezxmloutputhandler.php#L278 and https://github.com/ezsystems/ezpublish-legacy/blob/master/kernel/classes/datatypes/ezxmltext/ezxmloutputhandler.php#L285

But http://php.net/manual/en/language.types.array.php says:
Additionally the following key casts will occur:
Strings containing valid decimal integers, unless the number is preceded by a + sign, will be cast to the integer type. E.g. the key "8" will actually be stored under 8. On the other hand "08" will not be cast, as it isn't a valid decimal integer.

I.e. the string conversion is pointless, the array index will be an integer anyway.

PR: https://github.com/ezsystems/ezpublish-legacy/pull/1356