Article Argument: by taking only 1 level in wizard, you learn & cast every wizard spell, using scrolls to add spells.
Answer: No. On p44 (PHB 2024), it specifically says you "prepare" spells as if you were a "single-class" wizard of that level. So, you'll be limited always to only preparing 1st level wizard spells, no matter your available spell slots.
On p45, it notes your spell slots may be higher than what you can prepare. You might even get to a point that you have, for example, 3rd level slots and no class that can cast 3rd level spells. Whether multi-classing or not, you can always slot in a lower-level spell into a higher spell slot. Sometimes this boosts the spell, sometimes it won't. In this instance, you can slot in a 1st level spell, for example, to cast in that higher slot (e.g. boost burning hands to deal more damage) but you'll never be able to cast a wizard's fireball.
I don't see any authority that supports the author's claim that "scrolls" let you bypass the rules. That's nonsense.