Infusion mashing is a traditional method using an unheated mash tun, resulting in a single temperature mash. In reality, the temperature drops slightly over the mash rest and increases during sparging. Step mashing uses a heated mashing vessel, frequently called a mash mixer (or maisch böttich in Germany) and incorporates a number of rests. The rests in many step mash profiles coincide with the rest temperatures of decoction mash profiles. Step mashing is really an extension of decoction mashing.
The question you present relates to how these methods are practiced and when you ought to use step mashing rather than infusion mashing. My brewing professor and mentor says that mashing is really an extension of malting — what does not happen in the malt house must be completed during mashing. You are absolutely correct that well modified malt is great for infusion mashing. Well modified malt has had significant enzymatic degradation of the barley endosperm during malting and the starch granules are easily hydrated, gelatinized and broken down in size by alpha and beta amylase during mashing to produce wort. Well modified malts typically make for easy wort separation because they do not contain large molecules that are found in the endosperm of unmalted barley.
Malts that are not well modified are usually called “under-modified” or “poorly modified” malts. This terminology is vague, but the important thing to remember is that the malted grain still retains some of the barley endosperm character. In other words, the endosperm still contains large molecules of protein and carbohydrate gums that make the endosperm very hard. In extreme cases, poorly modified grains contain a “steely end” that looks like the steely (red-gray color) endosperm of barley. Fully modified grains do not have a steely tip and have a uniform white endosperm color. This is one reason why malt is evaluated by chewing.
Poorly modified malts do not work too well in an infusion mash because these gums make for difficult lautering. Low temperature rests of 118 °F (48 °C) allow the enzyme beta-glucanase to degrade beta-glucan gums (a type of carbohydrate) into smaller pieces. This is important since long beta-glucan chains dramatically increase wort viscosity and make wort separation difficult.
The other thing step mashing allows is for rests at the optimal temperatures for beta amylase and alpha amylase as well as a controlled temperature ramp from the beta rest to the alpha rest. This to me is the real key to step mashing because it gives the brewer the ability to increase wort fermentability. Most highly dry beers have a long rest at 140 °F (60 °C) for beta amylase and then a slow ramp up to 158 °F (70 °C) for the alpha rest. The result is an increase in wort fermentability as compared to infusion mashing at a single temperature.
So why does the Rockyard Double Eagle clone recommend a step mash? For starters, it’s important to point out that this beer contains about 20 percent wheat malt. This is an important detail as many brewers feel that a protein rest is necessary when brewing wheat beers.
Another point to consider is the quality of malt being used. In this case the recipe calls for 2-row barley malt. There is no connection between 2-row barley and well-modified malt. While it is true that American and British 2-row malts are typically well-modified (some would argue that traditional UK malts are over-modified), it’s also true that traditional “Continental” malts from Europe are the quintessential under-modified malts and just happen to be malted from 2-row barley. Six-row barley is a North American thing, as it is not native to Europe and continues to be grown only in Canada and the United States.
I would wager that your assumption that the 2-row barley malt used at Rockyard is well-modified is right and that almost any 2-row or 6-row pale malt produced in the US or the UK will work well in an infusion mash. But remember, mash choice goes beyond the modification topic when you consider such things as fermentability, other ingredients (such as wheat malt) and ease of wort separation.