An SLA of 100% just mean your account will be credited for any downtime. It doesn't mean that the company guarantees 100% uptime. No company signs a 100% or 99.99% SLA expecting to actually get 99.99% uptime but with the understanding they will be compensated when their is an issue.
None of the major cloud vendors actually hit 99.99% uptime.
Interesting distinction here: 100% SLA on responding to incoming DNS requests. The R53 console or management interfaces could be down and the SLA stays in tact-- if you can't update your DNS then 100% incorrect responses isn't very helpful.
By its very nature, an SLA of 100% is a guarantee that the service will be available 100% of the time or else the relevant penalties, explicitly stated or otherwise applicable, can be applied.
The question is whether the guarantee is meaningful by way of whether the penalties will significantly dissuade failures to meet the guarantee, and I'd argue in the case of Cloudflare, this isn't the case.
[Edit: Cloudflare's standard] penalty is a service credit defined as follows:
> 6.1 For any and each Outage Period during a monthly billing period the Company will provide as a Service Credit an amount calculated as follows: Service Credit = (Outage Period minutes * Affected Customer Ratio) ÷ Scheduled Availability minutes
And that's woefully inadequate for any enterprise client with mission- or life-critical services.
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TL;DR: A SLA is a guarantee, by the very definition of the word "guarantee," that a service will be delivered to a specific level and that certain agreed-upon penalties will be applied to the service provider if this guarantee is not met.
As you note, unless you negotiate a custom contract, usually the "penalties" are very, very mild. It's effectively the same as there not being a penalty at all. The SLA is just a marketing nice-to-have divorced from the engineering realities
Yup, and given Cloudflare's recent performance, I'd venture that more heavy-handed contracts need to be negotiated with them to drive an improvement in performance, or at the very least a paradigm shift in how they sustain availability to the clients who really care for it.
None of the major cloud vendors actually hit 99.99% uptime.