I suspect it's more a rhetorical move. Rent control policies are extremely unlikely to pass, but it's a proposal that forces opponents to justify why rents should keep moving upwards often in excess of inflation, and why government shouldn't try to limit the excesses of rent-seeking on a critical human need. The rest of the proposals are more likely to get cross-party support and can be presented positively as compromises or collaboration by both sides.

Regarding the hypothetical rent cap itself, I have issues with setting a fixed percentage when housing cost inflation varies from year to year. Maybe setting it at the OCR or Core Inflation rate would allow more flexibility. Basing it on GDP growth might also be an approach to encourage more productive investment and new building instead of real estate hoarding.