What is the fraction of the Bitcoin mining-power that might be engaged in Selfish-Mining?

Is there an estimate for the fraction of miners engaged in Selfish Mining within the BTC network? In light of the publication The Impact of Selfish Mining on Bitcoin Network Performance (2021), selfish miners not only gain an extra (unfair) share of the block rewards but also increase the chance of forking the network and implementing double-spending attacks. To accomplish that, a group of mining pools holding ~25% of the mining power would be enough to attack the network.

I believe this should get more attention from the community since it represents a significant cut in threshold-security from the famous 51% to about ~25% of the mining power.

It seems that extra elements could cut the threshold-security even further: e.g. If the miner engaged in selfish mining spread 0-power nodes around, they could use those nodes to communicate their selfish-mined blockchain (whenever it is convenient for himself) and leverage advantage on network visibility; meaning: the selfish-minted blockchain would have more visibility then a competing & fairly-minted blockchain.

Searching around the web, I could not find an estimate for the fraction of the BTC mining power that might be engaged in selfish mining.