Not necessary, because the argument is concerned with what occurs in front-loading washers. Even if some top-loading washers use different amounts of water from other top-loading washers, that doesn’t affect what happens in front-loading washers.

Not necessary, because what matters is whether a laundry detergent formulated especially for front-loading washers can dissolve readily in a front-loading washer. As long as it can do that, it doesn’t matter whether that detergent dissolves more or less readily in a top-loading washer.

Not necessary, because the author can believe that a top-loading machine can get clothes really clean with ordinary detergent. What matters is whether front-loaders need a detergent specially formulated for it.

Necessary, because if it were not true — if a laundry detergent CAN get clothes really clean even if it doesn’t dissolve readily in it — then the fact ordinary detergent doesn’t dissolve readily in a front-loading washer would not establish that a front-loading washer needs to use a different kind of detergent to get clothes really clean.

Not necessary, because the argument doesn’t depend on a comparison between whether front-loaders get clothes more clean than top-loaders. What matters is whether front-loaders can get clothes really clean only by using a special detergent.