61.129(a) Aeronautical Experience for a commercial pilot applicant says, under (3) "20 hours of training on areas of operation in 61.127(b)(1) that includes solo flight time...."
This is how the certification regs read. They always have. Each certificate requirement starts with the training requirements for that certificate.
I reference the private pilot requirement 61.109(a) "must log 40 hours that includes 20 hours flight training from an authorized instructor and 10 hours of solo training in areas listed in 61.107(b)(1)"
These solo hours are not just boring holes in the sky, but are supposed to be training and solo practice in the maneuvers listed which are more detailed in the appropriate PTS. The instructor who is endorsing the applicant is saying that applicant has performed the required solo training requirement.
So...to me, that means the solo x/c for the commercial applicant should be performing at the commercial level, ie, finding a dark grass strip in the middle of a dark and stormy night with no ifr assistance to pick up a paying passenger. More stress. More technical knowledge. More of what you would want your airplane uber driver to know about.
It is my opinion, but I got it from the regulations, and so do many others...of the old school, so that is what you will find with many of the old school instructors and examiners.