Keyword spotting - constant phoneme length
There is no relation between fixed frame length and varying phoneme length. Since frames are just 0.01 seconds and average phoneme length is about 0.1 seconds, there would be a frame located around a center of the phone. And in that point of time DNN detector that spans 20 frames around central frame will detect the phone of interest even if it does not have any time-varying architecture. The fully connected DNN they use could do that pretty reliably.

Of course they could use something more advance and adapted to varying length sequences, for example, multilayer bidirectional LSTM, that would result in better detection accuracy but at the same time it would be much more computation-consuming and thus battery-consuming, it would also affect reaction time, thats why they prefer simpler architecture.

Their approach would not work for some significant phoneme length variations. For example you can test that "heeeeeeey siiiiiiiiri" does not work that reliably. At the same time one could argue that "heeeeeeey siiiiiiiiri" is not the same as "hey siri" for a human either and should not activate Siri. For a balance between energy consumption and detection accuracy such simple solutions are a good choice.