thats closer to the opinion of the chazon ish, which is interpreted as anywhere between 650-950 liters, i dont recall all the different places ive seen the different numbers claiming to be the chazon ish's shiur
do u know what the name of the sefer/kuntres where he explains why rubber is a kli hamekabel tumah? over here it sounds like its referencing to somewhere
I now noticed an asterisk at the beginning of the piece, that mentions that this piece is a quote from טהרת מים by R' Nissan Telushkin. TLDR
However, regarding the rubber, this might have been a verbal instruction. R' Landa was the Posek in the courtyard of the Rebbe Rashab. IINM everything we know about the directives of the Rebbe Rashab as it relates to מקוואות are based on his testimony and writings.