I'm thick and don't get it, what's the takeaway here? What does this footnote add to what was already said?

Edit: Okay so I might know what you were getting at? unsure. My thoughts below, not necessarily all relevant to what you said

The 14th amendment is specifically called out in that referenced bill, not because it was the only one that was forced onto the readmitted southern states and hence uniquely undemocratic or whatever originalists think, but because it was at issue when the bill was passed. The 13th amendment was already in force well before this, and falls under the "said rebel States shall have formed a constitution of government in conformity with the constitution of the united states in all respects" provision, and the 15th had not yet been proposed when the bill was written and passed, hence why on first blush it looks like it was singled out, but actually it is not so unique. They went on to do the same to get the 15th ratified

the US was never an absolute or even remotely good democracy, so ofc I'm hardly going to ever be swayed by any appeals to "but this particular provision I want to strike down for nefarious purposes didn't have a democratic mandate or popular sovereignty or whatever way back when so we just have to take peoples (limited, wishy-washy) rights away now"

bonus fun quote about liberals when they get 1 (one) concession:

The amendment's adoption was met with widespread celebrations in black communities and abolitionist societies; many of the latter disbanded, feeling that black rights had been secured and their work was complete.