I think this link title is misleading. This isn't a revision of JSON but rather a revision of JSON Schema, a language for writing grammars that can be automatically validated.
JSON itself would not have if statements under this proposal, and indeed the syntax of JSON wouldn't change in any way.
I think this would be a very harmful change to JSON, but fortunately it turns out to be a change to JSON Schema, where it shouldn't really be a problem. (I think it's not a langsec concern because I think the same kinds of parsers should be able to validate data against these grammars with or without the if statement... depending on what kinds of state can be used in the condition, maybe.)
JSON itself would not have if statements under this proposal, and indeed the syntax of JSON wouldn't change in any way.