The clarification doesn't clarify; JC clearly does not understand what's being asked (the problem is not the contributions of each individual class, it's the fact that EK 4/wizard 1 has the same total number of spell slots as EK4).JC, when asked , "Should multiclass 3rd casters round up or down? Single-class rounds up, but multiclass rules says to round down." said the following, "A multiclass character follows the rules for multiclassing." If this wasn't clear, he continued- "this means that EK multiclassing wizard doesn't advance spell slot progression. Intended?" "An EK is at least a 3rd-level fighter and therefore contributes at least 1 level to multiclass spell-slot advancement."
However, I'll concede the point that the designers have had sufficient opportunity to rectify the issue, and have not done so, and therefore at this point it must be considered RAI as well as RAW. I still think Yunru's house rule is the way to go, though.