Not true. Washington was the most pious president and often used the specific name Jesus Christ, as opposed by the enigmatic "God" when addressing moral lessons.
Click here for the digitized papers of General Washington. They are searchable. When you search "Christ" the only things that come up are "Christ Church" (he attended Mass at several of these; the one in Alexandria being his home church which you can visit and see his family pew).
He most often referred to god as "Divine Providence" or "Providence."