FWIW it's not true. You don't lose trademarks by letting others use them, you lose trademarks by not paying your renewal fees (or genericisation: but that's actually protected against by trademark use).
This is not legal advice; this is my personal view and in no way relates to my employment.
Also not a lawyer, but it is either literally what that boss said or very close to what he said — he may have used a different expletive, but he didn’t seem to be a fan, and I have every reason to think he was sincere.
This may not seem friendly to you, but the default is a heartless C&D with the letterhead of a legal firm and starting something like “our client…”