‘...rent to homebuy allows you to rent the property for a maximum of three years’ is NOT the same as saying you will be permitted to rent for the full three year period and only then have to form a view about buying. The key is the use of the words "maximum of," clearly showing that the period allowed could be much less.
In terms of law, OHG have complied. However, one wonders that it might be a bit sharp practice if the relevant documents weren't clear that they might do this even though it is legal.
Interestingly, the other RSL's quoted don't do this at all, probably because they are aware that although legal it is sharp practice, ethically questionable, and reputationally damaging, as the article highlights. Not smart at all; bit of an own goal in fact.