lead in concrete stains
Is it true? Try a lead testing kit, commonly available at hardware stores - while primarily intended to test paint, I'd have to think it would give some indication. For a more definitive test you can probably send a sample to a lab. Red Lead was a common pigment, yes. Whether or not that was what was used on your slab, only testing will tell.

Whether you even need to seal in the hypothetical lead is debatable - the primary concerns with lead paint are children eating flaking paint or dust - if the slab does have lead but is not crumbling away and being eaten or inhaled, there isn't a pathway for the hypothetical lead to get into you.