This is a very curious statement. Tomiki was adamant that the ma ai of Aikido precluded the execution of Ju-waza with his concept of rikaku taisei or waza that can be applied outside of Judo grappling range (iow aiki waza). Basically if one mastered the ma ai of aiki waza then one never gets into the range where close range hip/leg/shoulder throws and takedowns can be executed, which means that it will be very unlikely that one will end up in a ground grappling situation.
Tomiki saw Judo and Aikido as separate Budo sharing common principles. In that light I don't think he ever taught ground grappling as part of his Aikido, since this was strictly part of the Judo syllabus. If one wanted to grapple or do newaza one would study Judo, he did not mix them afaik.
I think if one is looking at Budo holistically one needs to understand ground grappling, but it is not necessary for the understanding of aikido waza if one has an effective training method imho.
As I have trained under Tomiki Shihan for many years (15), we did a lot of tachi waza and as we took them to the ground, we would go into ne waza---shime waza, kansetsu waza etc. Shiho nage would lead into Kata Gatame and then to juji gatame etc, etc. Tomiki Shihan did not mix the arts but you did train paralleling each art and work on the ability to switch from one technique to another.