Nothing adds up to the story of memory shortage: MacMini and MacStudio are a very small percentage of the total Mac Sales, so it's not that stopping their sales is having any sort of impact on memory availability for laptops.
MacMini and MacStudio are also a very small percentage of the total Mac
revenues so if you so have a chip shortage then you're going to prioritise making $1100 MacBooks over $600 Minis. So, for example, delaying manufacturing a new M5 Mac Mini until you've met the demand for M5 MBAs is perfectly plausible.
The M5 Minis, Studios, iMacs and a replacement for the M4 iPad are expected Real Soon Now, so production of the M4 models will be winding down - if not already stopped. Apple will have tried to anticipate sales so that stocks will last until the new machines are available.
Have to postpone M5 Mini launch because the M5 RAM needed for MBPs (or any other reason)? Result: you run out of M4 Minis before the M5 is ready.
Have to raise the price or change the spec of M5 Minis vs. M4 Minis because RAM and SSD prices have gone up (or, again, any other reason)? Result: you don't want to accept orders at the old price points that are going away, so you refuse orders rather than give free upgrades. Note: base MBA price has gone up with M5 although you do get more storage.
And the iMac sales are very much higher, yet it is available with any memory configuration.
So?
That could just mean Apple made more M4 iMacs before winding down for M5. Maybe the iMac launch wasn't planned until later, so they made an extra batch. Maybe the iMac is skipping straight to M6? Who knows?
For an international enterprise like Apple, higher sales likely make the logistics easier & demand easier to predict - electronics manufacturing is hugely sensitive to economies of scale. Also makes it harder to guess how many high RAM/Storage spec Minis to make, since they are a harder sell & you lose more if stock is unsold.
Supply, manufacturing and distribution chains are complicated and depend on a lot of things. Impending new models and industry-wide component shortages & price-rises are
both likely to be factors. People seem to be inventing a lot of simplistic false dichotomies here.