It seems that from this forum post, relaying an email, that Swensen would suggest using a combination for long-term treasuries and short-term treasuries (barbell strategy) with an allocation to mimic the market's average duration of treasuries.He has the calculation for that time, and it hasn't changed much.
https://socialize.morningstar.com/NewSocialize/asp/FullConv.asp?forumId=F100000015&lastConvSeq=53479
The easiest way to match the market would be to own some of the 1-3 Treasury (70%) and some of the 20+ Treasury (30%) so the weighted average duration matches the market's 5.07 years -
I was close to doing this already as I use 15% LTT's and 15% STT's to keep it simple. He doesn't seem to mention TIPS though. Any thoughts on that, SeattleCPA?
So, in Unconventional Success (in Chapter 3, page 84), Swensen after a discussion of how to construct a portfolio, says, "Table 3.1 contains an asset-class combination that serves as a reference portfolio for investors to consider."
So a first thing I'd point out is that language isn't exactly canonical.
And then that table he references, Table 3.1, shows U.S. Treasury bonds 15% and US Treasury Inflation Protected Securities 15%.
The example portfolio recipe, then, does use TIPS.
The book does not specify which U.S. Treasury bonds though. But online sources like the one you mention and then a Boglehead user who says he just emailed Mr. Swensen and received a reply indicated that you want the duration of the U.S. Treasuries in your portfolio to match the duration of the market. Further, apparently in both of those sources, Swensen gave an example of how to do that by buying X percentage of short-term Treasuries and Y percentage of longer-term treasuries.
What I do, in case you're interested. I just buy (and think I'm correct to use) the Vanguard intermediate term treasuries fund. Their duration roughly matches the duration of U.S. Treasuries. E.g. maybe intermediate treasuries show a duration of 5.2 years and then treasuries as a whole show a duration of 5.5 years? It seems close enough to me.
One thing I'll again mention. Because I have a compulsive personality. Swensen's language clearly doesn't say we must use the exact percentages from Table 3.1. He gives them an example. And he notes that, yeah, someone would want to nudge them around, following his thinking, for your personal situation. E.g., if you own a home, you maybe dial down your REIT percentage. Or if you hold a bunch of equity in a job or business, you maybe dial down your equity percentage.
BTW another personal note: I've always thought about nudging the percentages around because I do own a home and because I own a part of a small business. And, gosh, probably I should. And then I think, I'm probably close enough...
P.S. The Swensen percentages again for anyone following this discussion:
US stocks 30%
Developed markets (so international) 15%
Emerging markets (so international) 10%
REITs 15%
U.S Treasuries (intermediate term, I say) 15%
TIPS 15%