Some notes to pick on in that article
1 - i get the impression that pregnant females don’t get back pain…
I have yet to meet any female who does NOT sufer back pain.
mind you, i’m not stating the cause of backpain is from lack of Transverse abs
2 - The article implies the SIT up works the Transverse Abs - and that pregnant people cannot even do one sit up - Sit ups focus is the Rectus abs. It might not fully 100% isolate it - i don’t think anybody assumes it does? But majority of effort is directed towards it.
The fact that pregnant ladies cannot do sit ups, weakens their core - the larger belly pulls forward and Down - resulting in the spine being pulled in a similar fashion - thus causing Back pain. Resulting in lots of sitting down, causing their Hip Flexors to seize and or tighten and thus causing further Back pain.
The article baffles me saying their is little evidence to suggest being pregnant causes Lower back pain… Does he even know females?
Then the article contradicts itself saying after birth, Lower Back pain is reduced in the majority of women before 4-6wks of time.
He seems to allude that The abs are at 0% useful up until around 8wks after birth. Surely they develop progressively over time, and that even though not yet at 100% by wk 8, the strength they do have at say week 6 is still far greater than 0%, but less than 100%. Why cannot they be 80% strong, but at that 80% strength is easily strong enough to support and stabilize etc.??
For an everage sized female who is lets say 8month pregnant - have you ever felt her belly - Hard as a rock. Not soft and flabby. Just simply think about that and stability for a minute. It doesn’t take a brain surgeon to figure out. 
Then Obese people - esp males with the Large Belly - I have not meet one who does not have Lower Back Pain - in some form. I meet new people like this Daily. All suffer Lower back pain.
I’m up to The Timing Issue - but no longer have time to comment.
I’m under the impression this guy assumes one can Isolate the Transverse Abs - or that others think they can. Well, to a degree you can Focus on it, but nothing is ever 100% - It might be as little as 50%, but you sure can feel that muscle work.
I feel his interpretation of Isolation is to the extreme and puts others into that Extreme Basket. Or maybe i have more faith in other trainers than he does?
A set of super strong Abs - cannot outset a set of Super tight Hip flexors acting on the Hips causing Hip rotation thus causing Lower back pain.
However - with the tightness removed and flexibility resumed to the hip flexors, A set of Weak Abs will still cause the hips to rotate downwards and back pain will still result.
The body is one big unit - not a set of abs or whatever, they all tie in. From reading just that 1st part - the author is not leading me to continue reading, to me, he is implying the ABS do NOTHING.
Sure they are overrated in some circles - but he is implying they do nothing…