That's interesting! I know Brimski has the true Hummer's Giant from Charles Brewer, but the plant you have does not look like it. A big tell-tale sign of whether it's the true or fake HG is the size of the midrib running down the center of the plant. I've been growing the fake clone for three years and the real deal HG for around six months. Here are some pictures of my true Hummer's Giant, who I have traced the origins of back to Charles Brewer, who took over Hummer's collection:
the TRUE ceph 'Hummer's Giant' IMG_1354.JPG (2.49 MiB) Viewed 4747 times
the TRUE ceph 'Hummer's Giant' IMG_1356.JPG (2.43 MiB) Viewed 4747 times
Now here are a couple specimens of my fake Hummer's Giant. I recently divided my XXL three year old plant so have a lot of them but none super large.
the FALSE ceph 'Hummer's Giant' IMG_1357.JPG (2.31 MiB) Viewed 4747 times
the FALSE ceph 'Hummer's Giant' IMG_1362.JPG (2.41 MiB) Viewed 4747 times
Despite one of the fake HGs being much darker, I can assure that they're the same clone. Even though they have different color, both the midrib size on the fake HG clone remain generally the same. Which leads me to my next point- different growing conditions. It is possible that you do have the true HG, however you're growing it in different conditions so naturally the pitcher morphology is different. Still, even under very intense light like what I grow my cephs under, the giant midrib stays the same, which is why I don't believe you have the true HG. Here is a comparison between the two midrib shapes:
the TRUE ceph 'Hummer's Giant' midrib IMG_1366.JPG (2.16 MiB) Viewed 4747 times
the FALSE ceph 'Hummer's Giant' midrib IMG_1364.JPG (2.38 MiB) Viewed 4747 times
Additionally, the pitchers of the true HG are much longer compared to the fake HG.
The cutting you showed a pic of does not appear to be the true HG. However, the seller's other plant you shared a picture of looks like the real deal with the giant midrib. It's harder to tell with the mother plant, but it doesn't look like the real deal to me either. It's possible that Brimski got "Hummer's Giant" cephs from two different sources and that one of them is real and the other is not.
It would be interesting if you contacted Brewer and got a hold of the true Hummer's Giant and then grew it side by side with your one from Brimski and after a few months observed the differences
.