We can speculate what if. One thing for sure, Federer would not have 17 Slams in his pocket if he were playing during Lendl's generation...
Well, he'd likely have all of Lendl's US Opens, plus Becker's from 89, Sampras' first one, McEnroe's from 84, Connor's from 82 and 83 and
for sure Wilander's from 88. Give or take the odd anomaly Federer would have 7 or 8 US Opens.
One thing
for sure also is Federer would also have Wilander's 88 Aussie Open and probably his 83 and 84 wins on grass too. Kreik's two Aussie Opens would be Federer's too, as would Lendl's from 89 and 90. That's 7 Aussie Opens right there -
for sure - perhaps more if he snuck another off Edberg at Kooyong.
Wimbledon... you think a 17 year old Becker would beat Federer? Dream on. Or Cash? One of Edberg's wasn't particularly legendary also... That's 4 Wimbledon's right there
for sure without even going back to McEnroe or Connors' in the early 80s.
French Open? Well,
for sure he's got Noah, Chang and Gomez's titles without even needing to change his shirt. They were lucky chump champions there without doubt.
All this is without even being overly ambitious in the early 80s or commandeering either of Edberg's 2 Wimbledon titles or
any of Wilander, Borg or Lendl's French Open titles.
We know for sure,
for sure, that Federer would smoke a stack of majors off the historic title holders in the 80s. It's pretty much a given.
So, you're correct. Federer would not have 17 slams in his pocket if he played in Lendl's generation. He'd have 20 or more.