No, the egg scan was what gave those posters the idea/notion to decide that Sobble was promised to Ash. I recall way too many posts reaching that "conclusion" in various threads during that time frame.
This preview just solidified what those posters had already accepted as their personal truth, in the form of giving actually grounds to speculate. the possibility Basically, it was fanon ever since the scan, and the preview hinted that the fanon idea might be possible canon, too.
But make no mistake, those posters in question already decided Sobble was Ash's beforehand.
I'm not sure what you mean, the starters were already in merchandise long before we even knew about the egg.
Past trends always had Ash with a starter so of course they would think Ash would get Sobble/Grookey, it's clearly not like Ash getting Sobble is breaking trends here.
On top of that a lot WANTED the egg to be Sobble but EXPECTED it to be Riolu because they really didn't like Ash with Riolu/Lucario but they didn't actually think the egg was Sobble, only they were hoping the 0.01% chance it was going to be Sobble.
And quite a few people didn't think Ash was going to even get a starter or preferred he didn't, so the preview made it seem clear that Ash could be getting Sobble.
So I don't know how "Sobble was decided to be Ash's beforehand" by the viewers.
I question if you're mistaking speculation/desire/hope/wants with as you call it "personal truth" (or in the most sugarcoated way I can describe it "Confirmation Bias") because clear as day at the end of it all due to logic. Nothing is ever truly confirmed except on the (now) Sunday the episodes actually air (Thursday when Pokemon aired on that day). Remember, something horrific/tragic could happen and the episodes could just cease to exist and nothing could potentially air on a Sunday and thus will never actually be the case. I would refer this to as "worst possible case scenario" even if the egg episode actually had Riolu in the egg and it hatched at the end of the episode. The voice actors voiced it, it was entirely produced, worst possible case scenario happens and its never canon or never actually confirmed. Once it actually airs for the most part and "propagates" as it were, then its confirmed 100%.
Otherwise "development hell" or "cancelled development" is a thing.