You will not see Torval players Cmdr. In one expansion off the top of my head they hauled 600k merits to a single system being undermined around the clock without being seen. Any equivalent expansion by certain other powers would be an entirely different story. That said, fortification (as opposed to expansion) involves a small amount of merits that can be completed in a short period and you'll seldom coincide with enemy Cmdrs while doing it. So indeed it is very easy to be very active in powerplay but choose your targets to minimise contacts.Just to throw gas on fire—went to cause havoc in a peripheral Torval system for merits the other day. In open, naturally, fully expecting to get in a PvP fight or two. Never saw anyone else there, even when there was fortification progress overnight
Players simply cannot be everywhere all the time. The dangers of open are vastly overstated, you can always find a place no-one is paying attention to. And in busy systems, safety-in-numbers works for you. Even if you don't wing up the other players on your side will lessen the chance of you particularily becoming a target.
When someone (I assume it was "someone"s - people - because it was intentionally invisible, no-one knows how the accounts were controlled in fact) hauls 600k merits to a system it's because it really matters. And likewise the advantage that someone gets from doing so in solo/PG really matters too. Such episodes are perhaps the biggest reason why I think the modes need to be properly balanced.