Oh I see, hehe. thank you so much for the help!

You can. The guide just says that the method of trial and error is not reliable, unlike later in the game when you can get access to elm/irwin and roamers to check your delay. At this point in the game, you will just have to patiently do trial and error to see if you get your desired poke. In a sense, going for a shiny is easier to check since all you have to do is interact with the pokeball and see if the image displayed is shiny.

So you're saying I can't check what seed/frame I hit from the starter's nature/IV? :O

yep, I'd just save one step before you're forced to interact with the pokes. Also this from the 4th gen guide:

The Johto and Sinnoh starters also operate similarly, but it is NOT recommended to get perfect starters this way as you have no reliable method of confirming your seed or advancing your frames and will have to rely on luck from randomly moving NPCs. Additionally, if you're going for a shiny starter, you will be confirm if it is shiny by simply clicking on the Poke Ball and the image displayed should be the shiny sprite if you did everything correctly.

Hi, thank you for the detailed answer. What do you think the best way is to RNG for starters in my situation (just started new, with no access to Elm/Juggler call or roamers or anything), but I do have ways to check for IV. Should I keep trying and adjusting my seeds on the way from the results?