I would be interested if anybody could do a comparison between that PinLater and Celery. Feels like there is a bit of NIH involved, unless it's the 100K/sec rate is not something that celery could realize and PinLater can.
Celery has flexibility for message backends, but prefers RabbitMQ which comes with its own operations overhead, and does not have real horizontal scaling built in (it has failover techniques). RabbitMQ is great for mission critical messaging that survive system restarts.
My biggest worry is relying on Redis or MySQL. What happens if workers stop consuming tasks, wouldn’t that balloon the redis memory consumption pretty quickly? I suppose memory consumption is a factor of load and task bytesize. MySQL on the other hand feels like the wrong tool for this job.
You can write simple workers for Celery in other languages too, and there is one in active development for Java. But then Redis is not really a good choice, AMQP is more convenient for interoperability.
Have no idea if pinterest evaluated Celery, because they have not been in contact. If they did there are several pitfalls they could have gotten wrong at this scale, but I'm pretty confident it would have been more than suitable. It could have been a benefit for all of us and chances are they have underestimated the effort needed to maintain a custom solution.