If we take a look at Intel's shipping CPUs with core count of at least 24 cores and calculate a few things:
- their perf/w as cores*freq/TDP
- "24c TDP" as TDP*24/cores (slightly flawed due to uncore power, but taking it into account properly won't affect the conclusion)
- "24c 1.9GHz TDP" as 24cTDP*1.9/2.1 TDP:

model	c x GHz	TDP	perf/w	24cTDP	24c 1.9GHz TDP

8180 	28 x 2.5	205	0.341	175.7	133.5

8176F	28 x 2.1	173	0.340	148.3	134.2

8176 	28 x 2.1	165	0.356	141.4	128

8170 	26 x 2.1	165	0.331	152.3	137.8

8168 	24 x 2.7	205	0.316			144.3

8164 	26 x 2.0	150	0.347	138.5	131.5

8160T	24 x 2.1	150	0.336			135.7

8160F	24 x 2.1	160	0.315			144.8

8160 	24 x 2.1	150	0.336			135.7

8894v4	24 x 2.4	165	0.349			130.6	*

8890v4	24 x 2.2	165	0.320			142.5	*

* no DDR4-2666 support

and now a) look at perf/w of this custom 1.9 GHz 150W SKU: 24 x 1.9 / 150 = 0.304, which is lower than the lowest of production SKUs whereas its TDP is at least 5W higher than core- and clockrate-corrected TDP of production bins, and b) assume that actual dependence of TDP on clock rate is within linear-to-cubic dependence (so I'm taking the worst possible case against the point I'm making)

then the figures make it obvious that this "custom off-roadmap 24-core Xeon Gold CPU specifically made for our workload offering considerable value in Performance per Watt" SKU is apparently nothing more than rejects of parts currently in production which don't make the cut on power and/or working core count for production SKUs.

And considering Cloudflare's well-known stance wrt ARM vs x86 (see e.g. https://www.datacenterknowledge.com/design/cloudflare-bets-arm-servers-it-expands-its-data-center-network) I suspect -- assuming Cloudflare folks are negotiation-smart -- they got these rejects not just for free, but for a hefty subzero price, part of their deal being this blog post mentioning they're upgrading their fleet to an off-roadmap/custom Intel CPU "offering considerable value in performance per watt".

With Centriq, ThunderX2, and finally Ampere eMAG servers available (https://insidehpc.com/2018/10/ampere-augments-arm-servers-new-developer-platform/), I guess the value of such PR project targeted at various business execs is well worth selling rejects at negative price to prevent someone size of Cloudflare switching to ARM.

Only for a while though (or just for a few of their sites perhaps) -- Cloudflare is clearly going ARM given what they publish, and they are not the only one switching, it's well-known that MS is going ARM as well (e.g. https://www.datacenterknowledge.com/archives/2017/03/13/arm-servers-can-soon-power-half-of-microsoft-data-center-muscle).