I recall Barton playing 5 Pios into T2 against Brits. The map was Arnhem Checkpoint.
Although I am 99% sure he was trolling, I would say that this build is viable this map due to the following:
- Buildings. Pios can hop in and out of garrisons, decreasing the amount of distance they need to close. Enemy use of buildings can be avoided in the early game and can be countered as soon as the first flamer Pio hits the field.
- Relatively close High Munitions Point. This guarantees an early Flamer (and multiple flamers later on) which is crucial to make up for your lack of firepower. Flamethrowers are especially effective against Brit IS as they rely heavily on cover.
Imperial Dane is another person who goes 2nd Pio. The benefits are largely due to the extra capping power early on, but I would say that they so combat ineffective without Flamethrowers that it's almost always a better idea to pick up an MG42 as your second unit.
More Pioneers later on in the match is good for the repairs, which is something that WM sorely needs.
Either way, pulling history into an argument about thematic doctrines is a bad, bad idea, as evidenced here. If you'd like a more overt reason, it's because nobody ever agrees on history.