It's not just to do with shirt sales, no. It's generally a pretty good indicator of commercial
I don't agree.
Liverpool will sell far more shirts than City, their global fan-base is far bigger and therefore, their shirt manufacturer will have to pay more for the rights to the shirt-deal.
On the other hand you cannot rely on on Liverpool being in the CL, nor can you rely on them being a PL title contender. If I was a shirt sponsor I would value the global TV exposure (assuming I am a global company) of being a CL regular and PL title contender, such as City, over the fact that Liverpool are a more popular club. Does it matter to Standard Chartered or Etihad that Liverpool sell more shirts in Cork or Tromso than City? This ignores many other factors: what markets are they aiming it? what values do they associate with a club/brand? ..etc. Hypothetically, if your club sponsorship was to help your Chinese market, an immature market (with regards to football) and one where they like to associate with a winner, which club would be more valuable: Liverpool, Spurs, Arsenal or City?
I'm no expert on this, I suspect you are not either, but to say shirts sold equates to commercial attractiveness is surely naive and simplistic.