Continental drift is not about coasts "growing" at all. Continental drift is exactly what it sounds like, and should therefore be self-explanatory. The "continents" are "drifting" (moving in other words). With some simple research, you would have found the definition if you couldn't deduce the meaning. US Geological Survey
Coastal erosion is where the "coasts" are "eroding" away. Ocean waves and currents are washing away soil and rock, moving the coastline (where the water meets the land) inward in some areas, outward in others, and both in some locations, which is also known as submersion.
Thus we see, your original post was wrong, and the article you linked to had nothing to do with the parent comment, or in fact, your own comment. The parent comment, and indeed this whole slashdot post, is about Sea Level Rise. Your comment was trying to compare continents moving, with an annual change of several feet, not in sea-level rise, but in coastline erosion. Literally 3 separate topics.
Lastly, your sentence:
When it comes to coastal issues, a 3.5 inch sea rise in 50 years is relatively small.
is confusing as you're trying to say that a small amount of sea level rise doesn't matter very much towards coastal issues, which is the opposite of what this slashdot post is about. The sea level goes up, and the coastline moves inward. Not only from erosion (soil washing away), but because the water is moving further inland as it rises. Therefore, low-level areas will be submerged in water.
Sea level rise is not a relatively small coastal issue to an area like the the Maldives, which has an average ground level of 59 inches (the planet's lowest country). The sea rises, and not only does their coast disappear, but their whole country. That's kind of a huge issue.