... the most terrible ritual known to me demands only a rope which has hanged a man and a sword which has slain a woman; and that for a ritual which promised to summon Death itself - though what is truly meant by that I do not know and do not care to discover, since it was also said that the counterspell to dismiss Death had been lost.

• Chapter seventy-nine, HP:MoR

I can see your interpretation, but Quirrel's commentary does not lend it credence. He does fear Dementors, or rather their effect upon him, but - no, your interpretation holds; regardless of how high a probability there may be of the spell merely summoning a Dementor, the inability to dismiss what's summoned leaves too high a risk of freeing something worse.

Though I would think Quirrel would clearly express those concerns over opting to speak warily and vaguely of the ritual - and I think it out of character for him to not have thoroughly contemplated it.