Flamestrike
Legend
That's not actually a rule. In fact, I'd call it little more than wishful thinking on the designers' part.
Many forumists will vouch for that long adventuring days happening not very often. Many DMs will tell you it's incredibly difficult to railroad the players into keep going even 6 encounters that day.
Your other advice is sound.
Its not railroading anymore than building an encounter using the XP budget and CR guidelines is railroading.
Its not hard to hardwire into an adventure a hard (or soft) time limit or constraint. And as long as you dont do it all the time (I aim for around 50 percent of my adventures) then its not intrusive at all.
50 percent of my adventures feature a 6 encounter/ 2 short rest adventuring day assumption. Some of them feature just the one deadly encounter. If you enforce the 6-8 AD paradigm around 50 percent of the time, your players will naturally conserve resources around this expectation. The system becomes self reulating, and you can back off with enforcing it.
Its an art as much of a science.