Is there a “kern” table declared in the font?
Even though all fonts should have their kerning implemented in the GPOS table, there may be kerning info at the kern table as well.
Some applications such as MS PowerPoint require kerning info on the kern table. More specifically, they require a format 0 kern subtable from a kern table version 0, which is the only one that Windows understands (and which is also the simplest and more limited of all the kern subtables).
Google Fonts ingests fonts made for download and use on desktops, and does all web font optimizations in the serving pipeline (using libre libraries that anyone can replicate.)
Ideally, TTFs intended for desktop users (and thus the ones intended for Google Fonts) should have both KERN and GPOS tables.
Given all of the above, we currently treat kerning on a v0 kern table as a good-to-have (but optional) feature.