OK, I watched the video.
It consists of two parts. The first part talks about General relativity and the introduction of a cosmological constant, which from the argument should not exist in completely empty space.
He then goes to the Quantum Field Theory vacuum which has the continuous creation and annihilation of all possible fields of virtual particles all the time, and illustrates it with the proton. His discourse assumes that the proton is made up of three quarks and the rest is empty space. The theory I know does not say so, it says the rest is a gluon to quark antiquark and back sea, that holds everything together to form the proton. It is not empty space because energy exists within the proton, it is not zero.
So the presentation is incomplete and seems to me misleading, if we are to project the inside energy momentum conditions of a proton to cosmological scales and the cosmological constant. They are not the same.
Anyway the argument he seems to be leading to is incomplete.
If mass is associated with spacetime then wouldn't it be the mass associated with spacetime which waves in a double slit experiment?
In the double slit experiments, mass does not wave. The elementary particles are point particles as far as our experiments have explored, when they appear as particles, the appear at a specific (x,y,z,t). What "waves" is the probability of finding that particle at a specific (x,y,z,t) which probability is calculated by squaring the quantum mechanical amplitude describing the "particle/wave" entity which probability shows interference patterns in collective observations at double slit experiments.
In my opinion, until we have a solid theory which quantizes gravity and includes the standard model of particle physics speculation about how fields appear in cosmological terms is not productive. We have to wait for a theory, and a string theory seems to be the only candidate that can do this , to examine the cosmological constant of classical general relativity.