I think that the advice you are getting is to help you not be frustrated. I don't think anybody who has posted stands to make any $ off the purchase of an rodi filter.
There are a wide range of hobbiest on this forum. Some ballers and some on a budget. I myself definitely have a modest aquarium compared to a lot of folks on this site. I have limited resources so I want to make sure what I spend $ on has the most impact. I read forums for about a yr before setting up my tank. Countless threads about issues that could very easily been fixed by using an rodi filter. If you read any tank of the month threads on this or other forums you will not find a single one that does not use an rodi. If you look through tank builds of systems that really thrive, they all have different lighting, skimmer or no skimmer, different pump configuration, use a fuge or not use a fuge. But they all use rodi. There has to be something there.
My rodi was the first piece of equipment I bought. You can find fairly affordable new ones from Brs or I think you can sometimes find the base model spectra pure for $139. Another good resource is Craigslist or if you have a local club. Just make sure if you buy used that the filters are pretty new. Not a deal if you pay $70 for the rodi but have to spend $40 for new filter media. I'm sure it is possible to be successful wo an rodi, but it sure improves your odds

. And that is what it is all about. Improving your odds. If you are considering a $100 refractometer, I do think it is sound advice to forgo or get one much cheaper and get an rodi. In my mind it is the base piece of equipment.
Fwiw I use a hydrometer. Never had any issues.
Go slow. Don't get frustrated. You don't have to take the advice. Clearly everything is not an absolute. But, I'm fairly sure that while many people on this site did or are going to college, there are many very experienced reefers that do not hold a degree. Studying marine biology does not make one an expert on home aquaria.
Happy reefing!