That can't be the reason, you can't create water anywhere except where water spawns normally. So you can't surround yourself with more water and land/water layout you could make with destructible landfill is already possible to do with careful landfill placement, it only makes misplacements and regrets permanent or forces you to reload an earlier save to get rid of it.
I'm of course working under the assumption that you wouldn't be able to mine landfill from grass tiles to create more water than you spawned with, which would have balance implications. But as long as you can't, balance is pretty much not affected by minable landfill at all.
You could maybe argue that if you connect an island to main land with landfill bridges then you can be safe if you disconnect rails and landfill bridges. But you can make walkable 1 tile water with landfill as it is now which belts and bots can pass. You can't connect rail that way as it is now, but the difference is pretty much negligible. And you would have to manually disconnect the land bridge anyways. And at that point you are locked out of your main land rail system anyways until you reconnect, which is a terrible situation to be in which is why no one (cue the exceptions) does that currently. Of course a temporary measure to save your main base might be worth it, but you are also then much more limited in where you can build that kind of base.
And a land bridge 2 rails wide is extremely easy to defend regardless, I don't see a case for this extremely minor cheese among many other more impacting ones should have a such dramatic anti-measure "fix" when it negativly affects gameplay much, much more.
