Twilio's write volume is on the order of tens of thousands of writes per second. https://twitter.com/dN0t/status/360119871318659074?p=p
But this issue seemed to be with processing payments vs. metering usage, quite a difference in volume.
I haven't re-read the article, but if I recall correctly the Redis in-memory data was being used to keep a working copy of the current account balance (not the canonical accounting/bookkeeping version) and it was the results of calculations based on the Redis stored data that was triggering the payment processing. So I think it _was_ a problem with a "metering usage" scale system, which cascaded into the payment processing system working as intended, but with erroneous data.
Telco billing systems are (or at least used to be) one of the busiest uses of databases. When you're billing in increments of only a few cents per transaction (typical for outgoing SMS), the volume of transactions for any reasonable amount of money can be _huge_.
I don't know how soon Twilio are going to get within an order of magnitude or two of AT&T or Verizon, but I'd hazard a guess that their transactional billing database is _very_ busy by just about _anybodies_ standards.