It comes from the huge contradiction that appears if you don't assume he used his lifeforce.
I'll put it in a very simple, understandable way.
- Escanor in chapters 230 and 231 was only recorded as having a
power level of 114k and was
getting destroyed by Meliodas with a power level of
142k.
- Ludociel in his true body was
barely blocking an attack from a casual Demon King possessing Meliodas, who is weaker than when he's possessing Zeldris,
as confirmed by the DK himself.
- Escanor could
physically contend with and
damage the DK even before the One. He also
survived a magic attack from him.
So therefore, if we assume that Escanor was not using his lifeforce, then the scaling becomes like this:
Right before noon Escanor < 142k Meliodas < Ludociel in a vessel who is 201k < True Body Ludociel < Demon King Meliodas < Demon King Zeldris Form 3 ~ Right before noon Escanor
Do you see the contradiction?