I actually consider this to be incorrect, and the reason is that the defences were large but not deep. The size of the area covered by fortifications is actually the denominator in the calculation.
Sevastopol's fortifications, which carried something like half as many guns (at least 500) on less than a sixth of the frontage (5 miles versus 37), were almost certainly more formidable. This is before getting into the specific issues with the Washington fortifications themselves, which were noted by contemporary engineers (one British engineer in late 1862 rode a horse up the sides of one fort to prove his point that they weren't impassable).
On top of that there's the siege gun issue. The Washington forts generally contained few to no weapons that could equal or exceed the range of even Union heavy siege rifles, and contemporary breech loading position or siege guns in some European armies would have been able to overwhelm any one fort. Since in most places the forts were not properly mutually supporting, the destruction of one or two forts would have rendered the defence scheme untenable.
The strongest section of the Washington forts is south of the Potomac, where they are better laid out for mutual support. Depending on the time period I believe there are actually spots you can march through the defences north of the river without being in view of any fort at battle range.