I believe he thought he needed to choose weather appropriate tires. And rain seemed to be randomly popping up an awful lot in races (probably multiplayer).
He’s not slamming you about the rainng in races, he’s merely pointing out that you are racing in the rain, yet every other day, someone complains there’s never any rain in the races anymore
Homologation will limit you to a certain Tyre Compound (Street, Sport, Race) regardless of weather.
When you are racing on a Rain variant of a Track your car will automatically be equipped with the wet weather variant of that compound.
Please note that there are 3 potential Scenarios for tracks to be in in Forza Motorsport 7:
Day - This is always guaranteed to be dry weather.
Night - This is always guaranteed to be dry weather.
Rain - This is split into 3 sections; the start, middle and end of race. Each of these can be one of 16 different severities of weather (from Cloudy all the way to to Rain + Lightning) with varying probabilities. The “wet version” of your chosen Tyre Compound will be used for all of these.
In short, don’t worry about it, the game will do the wet weather conversions for you.