Also, it doesn't seem like your description of how to do it is very accurate if I found what you meant (rather than running at a high speed, it required coming to a complete stop and moving slightly left on the sinking platform).
Have you got an input recording of this? I'm interested to see how you've done it - I recall just holding right the whole time and being zipped left to land on the checkpoint platform.
This is how nitsuja did it: http://www.soniccenter.org/eredani/LR1%20Floor%20Glitch%202.zip
The way we know of perfoming the glitch, you charge a spindash before the downhill, and by rolling into the sinking platform in a certain way you'll be pushed into the ground. It seems that you need to move at a slower rate in order to fall into the ground, as I've never managed to do it with a full-speed spindash.
Lava Reef 1 floor glitch, Savestate
There's one more thing that you didn't do, in Hydrocity 2 when you are at the section with the crushing wall, it's possible to warp into the floor if you spin and then jump against the advancing wall while facing in the opposite direction, then doing an insta-shield. You'll be pushed into the ground and can warp to the right, cutting about 10 seconds off the time.