I suppose by the technical definition the year end bonus cash is deferred comp, but I don't think 1 year is especially hard to plan around. People might stick out an extra few months because of it but it's not locking anyone in for years at a time. Plus even if you subtracted an entire year from the JS retention rates they are still quite good.
Regardless, the amounts cited by the original comment are only the base which isn't deferred by more than 2 weeks at a time, whatever your bonus potential is.