Whether learning scrolls is based on your total spell slots instead of your wizard level in PnP is discussed in several places on the internet, for example here:
https://rpg.stackexchange.com/quest...of-wizard-copy-any-wizard-spell-they-finThe original PHB is a bit vague, but it has been clarified since: which spells you can *know* is based on your class level, not your total level. Thus a Wizard 1/Sorcerer 19 could prepare Magic Missile from his wizard spellbook and upcast it using a level 9 spell slot, but he could not scribe and prepare a Wish (or even a Mirror Image) spell.
There's some technical discussion about the character technically being allowed to *scribe* the scroll, but not being allowed to prepare the spell. Which technically works out the same way.
This being said, it should be clear that a wizard 1/cleric 19 having access to all cleric and wizard spells can't be the intention.
