"236 Euro vs. 35 Euro; seems like a no-brainer for such a short ride."
236€ vs 35€ would be a no-brainer, BUT, it's 49€ vs 35€!!! Munich to Salzburg is considered a short trip (less than 250 km) for the Savings Fare; the fare starts at 19€ for the first person, 10€ for each additional person, 49€ for all four on an express EuroCity train. All four in 1st class is 59€, only 10€ more for all four.
With the EuroCity train, they can get to Salzburg at 10:00. The first regional train after 9 AM, an "M" (Meridian), doesn't get to Salzburg until 7 minutes before noon, almost 2 hours later.
The earlier Meridian leaves the Hbf at 8:54 and gets to München Ost just after 9 AM. If they were staying near München Ost (there is a Marriott Courtyard just down the street), they could walk to Ost and take the 9:04 M, use the Bayern-Ticket, and get to Salzburg just before 11 AM. If they weren't near Ost, four single trip tickets in the inner zone would cost them 10,40€ more.
So, for 45,40€ they would get to Salzburg at 10:55 with no possibility of reserved seats. For 49€ they'd get to Salzburg earlier and could reserve seats if they wanted to.
I think that is the no-brainer. But, they do have to commit to the 8:18 train since the Savings Fare tickets are train-specific.
Note: David only asked about a ticket to Salzburg, so I assume it is a one-way ticket. If he were making a round-trip in one day, the Bayern-Ticket and regional trains (still only 35€) would start to look better.