I have to disagree. Sure, I can build a character that "at level X only" may be better, not having to worry about getting there or advancing past that point.
But when making a character I am playing over the course of a campaign I don't think that the general statement of "the same multiclass character will be better then a single classed character at levels as they advance through levels at every level of their advancement except for levels 1, 5 and 11".
So I agree with what you specifically said, but think that's applicable to a one-shot and not applicable to a campaign.
i will give you and example for my case. maybe it applies to your example, maybe not.
Gm limited me to PHb only.
Character is a sorc and worked her way into being an entertainer as the concept of the person evolved in chargen and fiddling. So a young halfling who focused on perform, deception, persuasion and acrobatics and who sings her spells and "dreams with dragons" etc kind of carved her way into my existance from the early embittered senior urchin con man she started as.
Looking at the development, sorc thru 5 seems obvious. Get her to the second tier and early major unlocking.
But i wanted to do more, to bring some of her other aspects (singing or dreaming) into the mix after she hit that power, new power level.
So one thought was multi-class into warlock - chocing fey to match the wild magic. that would have been a two level dip at least to get to invocations and two slots for short rest.
but another idea which really seems to be the lead is bard for one level.
Delays a sorcery point.
Keeps the spell slots advancing but with a 1 level dip in new spells (gain new ranks spell at even not odd, but get slots at odds.)
Also gain a couple more cantrips, four more known spells (1st) giving her a good variety of the first levsl to cover more utility than just core focus - and a skill and light armor and an extra hp and a new (second) instrument.
trading off one sorcery point at 6th and delaying the arrival of spells at higher levels (but not slots) by one level does not seem to me to be a bad trade at all.
i doubt by levels 9-15 i will be noticing the one sorcery point much at all. My bet is i will see benefit from the inspiration dice, the armor Ac boost and the extra utility spells and cantrips much more.
i dont see this as a problem game-wise, it seems like a good use of multi-classing to chose to wed a little bit of a broad class with a lot of a more narrow focus class to get something between the two that fits the overall concept.
Even have another bard to work with in the group as we develop to sort of "share songs" between now and then and since she is going sword bard (ie likely spending her inspiration dice for personal damage gains) i wont even be stepping on her toes with my PHB sorcer-bard.