Something being "open source" does not imply anything about its license.
Suggesting that only GPL-like software is allowed to use the term "open source" is crazy. It doesn't reflect the world we live in at all: there are thousands of projects on github without a license, or with some "free for non-military use"-type of license, or MIT/BSD-like licenses...
EDIT: "the obvious meaning for the expression “open source software”—and the one most people seem to think it means—is “You can look at the source code.” (Richard Stallman, GNU Philosophy: https://www.gnu.org/philosophy/open-source-misses-the-point.... ). Not that Stallman did much better with "free software", which as the article argues, has the obvious meaning that 'the software is "for free"'.
Having to avoid expressions like "free software" or "open source" because two organizations decided to appropriate common english expressions to give them a complicated meaning is nuts.