If this wasn't intended to be a sudden tempo change, shouldn't it say poco - a - poco - accelerando or something like that? Or is it one of those things that are implied and it'd have to say subito for it not to be interpreted gradually?
I haven't looked closely at the example you gave, but in general I would say no, it doesn't have to say poco a poco or anything else. See, we are spoiled and in some cases molested by contemporary composers, or by composers from around the early 20th century, who tended to write exactly what they meant at all times.
Take Bartok for instance. He had exact definitions for staccato, portato, and legato. By exact, I mean he had mathematical definitions for them. They were objectively outlined in his aesthetic. Does that mean we should play articulations in Chopin the same way? Obviously not.
Also, Bartok, when writing out accelerandi or ritardandi, actually wrote out metronome marks along the way to show how it went - exactly and mathematically.
Lots of composers after him did the same. We're rarely taught now, that composers before that, often didn't notate things to such an exact degree, and markings required the input of whomever was playing the piece. Simply put, you can't approach every composer in the same way. Just because Bartok specifies to the exactly number of ticks per minute how fast or slow each bar is, it doesn't mean Chopin thought the same way.
That's why I said, interpret, interpret, interpret. What is the drama you are getting at? You have to use the marks to achieve that feeling. You can't just assume Chopin or anyone else was writing along the mindset we have today, which is, "Does he mean to do A or B?" In fact, Chopin allowed lots of different variations in markings to be published in his works. He saw them, didn't correct them, and didn't really care.
Do not apply an objective, modernist style, to subjective, romantic music.
Walter Ramsey
Topic: 'a tempo' marking