Sorry Jim but it is. See picture below. The H28 will get you close to both Isleworth station and step free Osterley Station on London Underground within ten minutes.
I tried to check the journey time between Chiswick station and Acton Town station (the nearest step free station) yesterday and it directed me to get off the E3 (the only bus serving Chiswick station) and it advised getting off a Chiswick Town Hall and walking (for 24 minutes from there). This was for a total journey time of over 40 minutes.
Michael is setting up a straw man argument here. Yes the area around Chiswick station is not going to see level of building that other areas will and yes the current usage is less than these other stations. However, if Brentford, Syon Lane, Isleworth and Osterley weren't step free, the users of those stations would have a nearby viable option. The latter three have all been made step free in the last five years. Disabled people living around Chiswick station have no viable option for accessible public transport for most destinations and the situation is little better for those living across the rest of Chiswick.
The question remains, why has accessibility provision in this area been consistently provided in areas which already had options for disabled people and not where there were none.