The common definitions have to do with stealing, but an equally valid definition of the word hijack is to:
> take over (something) and use it for a different purpose.
Taking over a project so you can have it to advertise your service is exactly that.
You are misleading readers in order to promote your agenda. You clearly speak perfect English, so you know what hijack means. "take over (something) and use it for a different purpose." is not found as a definition of "hijack" in any dictionary. "Hijack" implies "unlawfully" or "without having a right to do so".
Of course, every word can be used in a slightly different meaning; for example, in software can (harmlessly) hijack an entity (circumventing the usual API for expediency or performance). Such broadened semantics is perfectly fine when there's no confusion about the meaning. Very clearly in the case of OP, there was a clear intention to imply "unlawful" or "without having a right", so this exception doesn't apply.
The sad thing is that I actually support your agenda. I just don't support promoting it through misleading statements.
> many programmers would consider [this] a violation of the open-source spirit.
Instead of making the untrue statement above, just say
"They used, in my opinion, an unethical way to advertise their product; specifically, they bought OSS products and put their ads in there."
