Marxism is incoherent so debating it is largely pointless, but in the quotes above he clearly defines labor as a "standardized unit" which differs only in duration and intensity. This is nonsense so it's not surprising he later tried to walk it back a bit, but as you note, that was half-hearted at best and he usually treated labor as if it were something like coal.
In reality there's already a standardized unit which sums up the worth of a person's labor (money) and a standardized way to compute it (the market). Marx didn't like what that system computed and argued strongly that it all be swept away and replaced with some notion of "need" (family size) and "ability" (simple hours worked or intensity of those hours) which is drastically too simple to reflect reality. That's why his ideas led to collapse everywhere they were tried.
In reality there's already a standardized unit which sums up the worth of a person's labor (money) and a standardized way to compute it (the market). Marx didn't like what that system computed and argued strongly that it all be swept away and replaced with some notion of "need" (family size) and "ability" (simple hours worked or intensity of those hours) which is drastically too simple to reflect reality. That's why his ideas led to collapse everywhere they were tried.