It could be construed as an insult, but not as a personal insult. It is you who drives the tone of your "debates".
Let's assume for the sake of discussion that you have 50 years of extensive tennis knowledge -- this does not cure your general lack of knowledge of the effects of doping, and your lack of ability to demonstrate that doping causes or can cause the effects you observed.
Skepticism about Nadal from anti-doping experts, from professionals on the tour, and from top WADA officials speak to prevalence and personal faith, but do not bring, nor rely on, knowledge about the effects of doping on performance. This is why you say "they believe Nadal dopes". These expressions of belief carry very little substantive weight.
It is one thing to say that Nadal dopes, but it is quite another to say certain achievements "show doping". What I bring to the conversation is skepticism to your mythical claims like "only doping prolongs careers", and "the defeat of a 33 year old by a 34 year old, on clay, by the best clay player, can only be achieved by doping, therefore Nadal shows doping".
If you actually had any fruit to bring to a fruitful debate, you could show this fruit. Instead you avoid the fruitful exchange when prompted to provide more substance, in favor of personal accusations questioning "ability to grasp" or diverging into some tangential obsession with farm animals.
If you want debates to be fruitful, you must stop building scarecrows, and stop insulting those who ask you to provide some fruit.