Subsisting (sinking) soil is quite typical for container plants, for just the reason you mention -- the organic matter component (compost in this case, mainly) is degrading over time, which is normal. As pores in between the soil particles shrink, the soil level drops and it also can hold water for longer periods before drying out. (Not always a helpful attribute, but this depends on someone's watering practices as well and how wet-sensitive the roots of certain plants are.) Potting soil that includes inorganic (not biodegradable) ingredients like perlite and vermiculite help resist this compaction over time. Rice hulls might resist compaction, but we have less research on their use thus far and how successful they may be at replacing oft-used ingredients like perlite for this purpose. Other additives that gardeners use when custom-mixing their own potting blend is fine pine bark, which also decays slowly enough that it helps to resist compaction. (Potting mixes used by nursery growers tend to rely heavily on aged pine bark for partly this reason.)

It would disturb plant roots to uproot the planting to add soil underneath them at this point, so we'd suggest avoiding that. Fortunately, the soil is not likely to keep compacting to the same degree, so the plants should not sink appreciably lower than they already are from now to the end of the season. While you could re-dose them once with more time-release fertilizer if they are nearing the end of their original fertilization period, the plants pictured don't appear to be nutrient-deprived, so you could probably skip it and they should be fine. If you do decide to fertilize, follow package label directions, and yes, apply the pellets to the soil surface, not on top of the foliage, and then water them in so they start to dissolve. (It won't look like they're dissolving right away, but small amounts of the nutrients leach out of the pellets over time and the pellets become more hollow, not shrinking in size, as their contents are used-up.)

Miri

No, do not add compost or any type of soil on top of the existing plants, even when they are dormant during winter; they should not be buried by any additional soil or it could smother and kill roots and crowns. Perhaps the person advising that approach was thinking that the planters had annuals in them (annuals are more commonly grown in containers than perennials, given the challenges of having perennials overwinter successfully), and would therefore be empty of plants between late fall and spring, or thought that the plants would be transferred to the ground to overwinter more reliably, leaving the container empty.

We see that in a prior question about these pots, you mentioned using 5 inches of mulch in the bottom. This layer decaying and compressing as it does so could be the reason the soil has sunk so drastically. For the future, it's best to use the same material (potting mix) all the way down to the base of a pot, and just add drain holes to the container itself, since trying to use gravel (or mulch in this case) to catch drained water in the base of an undrained pot risks drowning roots.

If you want to adjust the planting height back to closer to the rim, you'd have to pull the plants out, add more potting mix, and put them back in the planter. If they have rooted-in enough, this might be fairly simple in that several will lift up relatively easily as a clump, so won't need to be handled individually. If you try this, we suggest waiting until early spring, as doing this in autumn may put the perennial plants at greater risk of winter damage or death since their roots would have been disturbed quite close to winter dormancy. Or, you can try right now, so the plants have a couple months or so left of root growth they can accomplish (if they need to regrow from the almost-transplanting adjustment). Keep and eye on their watering needs in case the root disturbance makes them more prone to drying out.

Miri