I think he means that since the Hotel Monasterio and the H.B. train are owned by the same company, it's easier to get seats on the H.B. (it's not a large train and there aren't many seats. Frankly, the Vistadome is a nice train and a lot cheaper, for what it's worth). If you can swing it, seriously consider the Monasterio -- very unique hotel.
Just a tip: You don't need a week in Lima. It's a great city, with great food and people, but three or four days is plenty. The Marriott there is in a great location -- but also check out the Miraflores Park Hotel (right nearby). It's a big more elegant (and a swanky rooftop -- I actually saw Paul McCartney last time I was up there). Rates can be even cheaper than Marriott if you book ahead (though, just as often they're more than the Marriott). And some of the rooms (I think the junior suites) have great in-room saunas (i'm a bit hazy on this -- I was kind of drunk when I tried it). But you can't go wrong at the Marriott -- it's a nice property, and again, a great location. And it's far more "Americanized" if that's important. There are some nice restaurants nearby, too. But after a few days, unless you really like to eat, you'll probably have had enough. Also: While the Marriott is in Miraflores, a very nice, leafy, upscale neighborhood, the center of Lima isn't quite as lovely (read: hang onto your possessions). Best to go on one of the organized tours they'll hook you up with at the Marriott. Have a blast -- Peru is a fantastic place.