It isn't because of some purported Gandhian mindset. It's becuase India is in a pacing conflict with China and Pakistan, whereas China is in one with the US and historically the USSR. Assuming the Chagos Archipelago dispute gets resolved in the next decade (India and France backs it's return to Mauritius because Mauritius' police and military leadership are under direct Indian control [0] but the US prefers Chagos remaining under British control because we are closer aligned), India has no need to explicitly publicize ICBM capabilities that extend beyond China or Turkiye.
Additionally, publicly stating India has ICBM capabilities makes it harder to land transnational mining deals [1] because then discussions with Australia, Brazil, Canada, etc adopt a nuclear proliferation dimension as well as placing a target on India's private sector because of SpaceTech and DefenseTech's dual use implications.
[0] - https://thesecretariat.in/article/inside-raisina-hill-nsa-to...
[1] - https://www.ft.com/content/c5868e2f-8d19-4393-93be-2ad726a63...
If I, some rando on the internet, can theorize "that sure looks like it could be turned into a storable icbm that could deliver a nuke to hawaii or Tierra del Fuego or anywhere else on the globe" can make that guess, then any other major world power is likely far ahead of me in analysis.
Also well known that having the institutional knowledge and technical capability to build or adapt something in a fairly short time frame is almost as good as having the thing ready to go.
This is similar to how Japan doesn't have nuclear weapons.
Exactly. This is the norm.