Simple-and-implementable is something you can deploy in the moment without knowing things beforehand. If you're at a grocery store, you can ask yourself "is this really processed?" as a simple filter that will largely steer you right. Similarly, Michael Pollan's advice to "Eat food. Not too much. Mostly plants" resonates with a huge number of people because it's a simple, actionable, not very fussy way to approach eating.
Complex-but-accurate advice is stuff like 'let me look up if this follows the definition of tier 3 or 4 processed', or 'I have to have 5g net carbs today and this is 1.2g so I have 3.8g left'. This has a much higher cognitive load, requires a lot of research and pre-planning, and is unlikely to be implemented in the moment nor sustained long term. It's also a place where people can develop unhealthy psychological relationships with food and eating by being hypervigilant about complex rules.
Most people find their lives work best with simple-and-implementable rules of thumb for food.
And yet nobody said "avoid ultra-processed food as defined by NOVA". They said "avoid processed foods" and "eat whole foods". It's extremely generous to assume everyone reading the former translates that in their head to "avoid ultra-processed food as defined by NOVA". And the latter is meaningless.
Even if that is what they really mean, that's really surface-level advice. There are foods in Tier 4 that are just fine. There are foods in Tier 4 that are more nutritious than "whole foods" are. Note that if you add whey protein to your soy milk, you have just created an ultra-processed food!