True enough in general that most older houses are over radiated -- up to a point. Really old houses, for various reasons, tend not to be. The principle place I care for -- Cedric's home -- has a total of 1400 EDR in radiation. Cedric and the radiation -- fired at 2.75 gph -- can only just keep up when it's really cold out! (defined around here as -10 F).
The big wild card is infiltration on 2 story or taller homes. Retrofits such as venting a previously unvented attic (for the record, I think vented attic are a stupid concept built on a myth, that needs to go away) for example, can be a killer. As the home settles and window sashes wear infiltration can really pick up.
That being said, unless your really “on the edge” on sizing, the very limited run time for an indirect should have negligible impact. But you have to try and evaluate each home individually, but I wouldn’t use any hard rule.]
150 gallons of hot water use with a 70F temp rise, is 52 minutes at 100k BTU indirect input. That would be on hte higher end I think for water use in all reality. That’s 1 hour at 2.5 GPM.
Keep in mind that in your example, -10F is in fact probably 5F colder than design temperature and a home should start “losing ground” just a little since it’s a <1% condition. Design in SE Iowa here is 2F. In Michigan, despite being further north for example it’s temepered by the lakes so design was +5F or higher in some locations. Plus here the coldest days, Though windy are also almost always sunny during the day, so you recover by late morning.