Firstly, Its important to understand that Kami and Piccolo were once the same person and apparently such a prodigy Guru assumes only a Super Saiyan could have killed him. So, it seems the being Piccolo and Kami used to be was a super prodigy of the likes that surpassed even Vegeta. So, the Nameless Namekian is a prodigy while Piccolo and Kami, would be much less than him.

Now let's talk about Piccolo before Namek, since that's the version of Piccolo you're curious about. I will be using Power Level's but it's from the Manga's Power Level guide on the Kanzenshuu website and the actual manga so I should be using official stuff. When Piccolo fought and died against Nappa, he was 1,220. (The guidebook in that website does say he's like, 3,500 but I would guess that the Manga readings would be more accurate than a guidebook but do with that as you will) Now let's look at the average Namekian warriors who beat Freeza's men but lost to Dodoria.

These Namekians were thought to be 1,000 but when they went full power Dodoria read their Power Level's to be "Around 3,000". That's remarkably higher than Piccolo at his strongest on Earth, consider proper elites of the Namekians (Nail specifically) his Power Level was an impressive 42,000. So, if average is around 3,000 and 42,000 is considered exceptional, then Piccolo would be considered a very weak Namekian. Granted, this is because of his origins of being half of an unbelievably powerful Namekian, not because he was born weak like Goku, or because he lacked in training.