"Ranting sideways" doesn't appear to be a recognised idiom, so we take it at face value.
Dictionary definitions you should take note of:
- "In a devious, indirect, or roundabout way" - Oxford
- "in a lateral or indirect manner" - Websters
I would understand it to mean that the "ranting" was so frenzied it was not properly directed at anything or anyone. The recongised idiom "ranting and raving" carries a similar meaning - that the ranting is so angry it is uncontrolled. A person 'ranting and raving' may appear to be talking into the air, moving erratically, so that they are not looking or speaking directly at any one individual.
I should also add that "sideways" appears to be used more casually as an intensifier, perhaps with less regard for the above definitions. I can tell you that, here in the UK, we use the expression "raining sideways" to mean exceedingly heavy rain, although there is also some literal meaning to this as wind-driven rain does appear to come at you horizontally. It seems possible that could be one origin of it as an intensifier.