By Reddit ads allows for gambling related ads to be posted, however, it would seem they require that the ad be targeted only to gambling subreddits. This was unveiled when Reddit advertiser Mzeemaneno tried to post a gambling related ad and was told to make sure they are in only specific subreddits.
This is not something that the Reddit Advertising Policy states. In section 11 the policy covers Gambling:
11. Gambling and Gambling-Related Services
Advertisements for gambling and gambling-related services must:
(i) comply with applicable laws, regulations, and licensing requirements, which may include those applicable to the location of your business, those applicable to the location of your target audience, and those related to responsible gambling;
(ii) comply with industry standards, as may be reflected in the American Gaming Association’s Code of Conduct for Responsible Gaming; and
(iii) ensure minors are not the target of such advertisements.
This appears to be either some new requirement or it could be something the Reddit user themselves decided to do to avoid additional scrutiny.