Are X-ray telescopes with glancing angle surfaces basically "funny-looking" Cassegrain telescopes mathematically?
Short answer, yes, the x-ray grazing incidence mirrors set up like this still function like a Cassegrain to minimize off-axis aberration.  However, it is still different in that a classical Cassegrain uses a fast positive paraboloid and a slow negative hyperboloid.
The Cassegrain design combines the paraboloid and hyperboloid to minimize off-axis aberrations effectively. In 1952, Hans Wolter outlined 3 layouts of paraboid-hyperboloid mirrors which could be used in a grazing-incidence for x-ray applications.  The type 2 layout is most equivalent to a Cassegrain, using a positive paraboloid followed by negative hyperboloid, the application used here (in the XMM-Newton observatory) is a Type 1.  As a result, the remaining aberrations are slightly different, though still minimized rather effectively. Also notable, is that the focal lengths of the paraboloid and hyperboloid in the Wolter designs are of similar focal lengths.
Type 1:

Type 2:

https://www.cosmos.esa.int/web/xmm-newton/technical-details-mirrors
https://en.wikipedia.org/wiki/Wolter_telescope