Why is the IUPAC definition of adsorption so different from other definitions?
The IUPAC defintion isn't more limited than the ones you've read. Instead, it is more general.
Your confusion results from not being familiar with the term &quot;condensed phase&quot;. You think a condensed phase is a 'more condensed liquid', and consequently think IUPAC is referring to a surface formed between a liquid and a 'more condensed liquid':  &quot;where there is an interface of two liquids, one containing a solute, and one being more condensed&quot;.
No.  A condensed phase means a liquid or a solid.  Thus, when IUPAC says the following, what they mean is that adsorption can occur at liquid-liquid, liquid-solid, gas-liquid, and gas-solid interfaces.

An increase in the concentration of a dissolved substance at the
interface of a condensed and a liquid phase due to the operation of
surface forces. Adsorption can also occur at the interface of a
condensed and a gaseous phase.

Is the IUPAC definition completely general?  No.  It doesn't account for adsorption between supercritical fluids and solids. But it at least covers all the routine phase combinations.
As to the issue of whether IUPAC's defintion of solutions (which limits the solvent to be a condensed phase) contradicts its defintion of adsorption: While it could have been written more explicitly, a close read indicates there's not necessarily a contradiction.  Let's take each sentence in turn:
The first sentence concerns a liquid phase in contact with a solid or liquid surface.  Here the adsorbent is referred to as a &quot;dissolved substance&quot;, which would make it the solute within a liquid solution:

An increase in the concentration of a dissolved substance at the
interface of a condensed and a liquid phase due to the operation of
surface forces.

Note that the solute can be (prior to its dissolution) a solid, liqud, or gas.
The second sentence concerns a gaseous phase in contact with a solid or liqid surface.  Here no explicit mention is made of a dissolved substance:

Adsorption can also occur at the interface of a condensed and a
gaseous phase.

Thus we could parse IUPAC's language to mean that adsorption involves either the solute within a liquid solution that is in contact with a liquid or solid surface, or a gas that is in contact with a solid or liquid surface (where that gas could be pure, or part of a mixture).