Heker L'mata (Eruvin 5a)

The gemara in the second to last explanation of the machlokes between Abaye and Rav Yosef says that maybe they argue if the heker shel mata is like the heker shel maala or not. Since when is there a heker shel mata?? Until now we've been talking about putting something on the bottom to close up the hole so it would be less than 20 amos. It has absolutely nothing to do with a heker!?! Someone suggested that the gemara is just calling it a heker on the bottom for parallel language even though the purpose isn't to be a heker. That would be possible but I still don't understand the machlokes here. Why should the width of the raised ground be dependent on the size of the heker on top? I can understand if you say the machlokes is based on if you could stand under that or if the reason for the heker on top is really because of mechitza but those were the other answers. What's the svara of the machlokes here?

I don't know an answer to that question but I saw that the Rif understands this gemara differently. He says we pasken that when you put a lower beam because the beam was higher than 20 amos - that beam only has to be one tefach because hekero shel mata is like hekero shel maala. So the gemara isn't talking about raising the ground but is instead talking about lowering the beam (or adding a second beam). The Rosh and the Baal Hamaor don't like this pshat for a number of reasons and I have trouble understanding this pshat also but I think that maybe the Rif felt he was forced into explaining this line differently based on what we said before. If we're only talking about raising the ground then that's not hekera shel mata. The fact that it's called a heker must be because we're talking about something on top.