Keep in mind that multiclassing is an optional rule, and the rules for each class are not written with multiclassing in mind.
While the rules for multiclass spells known/prepared could be clearer, the example given makes it obvious what is the correct interpretation: a Ranger 4/Wizard 3 has 2nd and 3rd level slots, but the example explicitly states that the character only knows 1st level ranger spells. Any reading of the rules that would let the ranger know higher level ranger spells must be wrong.
So by the rules a cleric 1/wizard 19 can only prepare 1st level cleric spells.
You are correct. What you are missing is that the rules for preparing spells as a cleric (as quoted above) specifically tell you that you can prepare any spell for which you have spell slots, and the rules for multiclassing do not distinguish between classes for the purpose of determining your spell slots -- you don't have 'wizard spell slots' and 'cleric spell slots', you have spell slots.
So as a Wizard19/Cleric1, you prepare spells as a 1st level cleric who also happens to have 9th level spell slots. You are limited in the number of spells you can prepare (since that's based on your cleric level, as noted in the multiclassing rules), but not in the level of spell you can prepare (because you have the spell slots of a 20th level caster, not those of a 1st level cleric).
--
Pauper
This is a classic "rules lawyering/powergaming" tactic I like to call "intentional ignorance." The rules are quite clear, yet you reach for the clearly unintended AND unwritten interpretation that grants outrageously overpowered options by ignoring a) the specific v. general guideline, and b) the EXPLICIT multiclassing rules.
Obviously, no one can tell you ["you" = anyone suggesting the 9th level cleric spells are ok] how to play
There is no conflict here [in the book] except the one being created, specifically, by those who want to get outrageously broken power.
The rules are quite clear, yet you reach for the clearly unintended AND unwritten interpretation that grants outrageously overpowered options by ignoring a) the specific v. general guideline, and b) the EXPLICIT multiclassing rules.
The rules are pretty clear, I agree, but you're the one who seems to be ignoring them. In order to support the argument that a Wizard19/Cleric1 can only prepare 1st level cleric spells, you need to argue that the phrase in the multiclassing rules that says that you prepare spells as a single-classed spellcaster also says you determine what spell slots you possess as a single-classed spellcaster.
The rules are pretty clear, I agree, but you're the one who seems to be ignoring them. In order to support the argument that a Wizard19/Cleric1 can only prepare 1st level cleric spells, you need to argue that the phrase in the multiclassing rules that says that you prepare spells as a single-classed spellcaster also says you determine what spell slots you possess as a single-classed spellcaster. The problem is that the multiclassing rules don't say that -- there's an entire paragraph on how you determine what caster level you are for determining your character's spell slots, and it makes no distinction between your classes -- the rules even explicitly state that you can cast spells from any of your Spellcasting classes with the slots you gain from the Multiclassing Spell Slot chart.
In other words, if you are a multiclass caster, you determine your character's spell slots from the Multiclass Spell Slot chart; those are the spell slots your character has. You don't have a subset or different number of those slots only to determine what spells you can prepare -- those are your spell slots.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.